The "barbaric" act - using a neurotoxin that is classed as a chemical weapon - could only have been carried out by Vladimir Putin's government, they said on Saturday.
The poisoning "highly likely" resulted in Mr Navalny's death.
Sky News understands it is likely the toxin was manufactured in a laboratory rather than actually taken from the frogs
It is not clear how the frog poison - called epibatidine - was allegedly administered to the dissident, who had been in a penal colony in Siberia when he died almost exactly two years ago.
Indigenous tribes in South America are said to use the toxin in blow darts or blowguns when they hunt.
The poison - described as "one of the deadliest on earth" - is 200 times stronger than morphine. It causes paralysis, breathing difficulties and death.
Yulia Navalnaya, the Russian dissident's widow, appeared at a press conference on the sidelines of a security conference in Munich to announce the discovery, flanked by the foreign ministers of the UK, Germany, Sweden and the Netherlands.
The four countries and France have been working together to establish how Mr Navalny, 47, died.
They plan to submit their findings to the international chemical weapons watchdog, the Organisation for the Prohibition of Chemical Weapons (OPCW).
"It is hard for me to find the right words," Mrs Navalnya said in English, looking visibly upset.
She had been at the Munich Security Conference when the news of her husband's death broke on 16 February 2024.
"It was the most horrible day in my life. I came to the stage and I said my husband, Alexei Navalny, was poisoned. What could else happen with Putin's number one enemy in a Russian prison?
"But now I understand and I know it is not just words. It is scientific proof."
Russian authorities have previously claimed the dissident's death was not suspicious but had been caused as a result of "combined diseases", including an irregular heartbeat.
But Yvette Cooper, the British foreign secretary, said the UK and its allies had been working "with fierce determination" to establish what really happened.
It is understood that British scientists at Porton Down played a key role in the discovery of the frog poison scheme.
It was not immediately clear how they were able to obtain samples from Mr Navalny's body to test for the toxin, though Ms Navalnaya has previously said they were smuggled out of Russia.
The opposition leader was buried in the suburbs of Moscow in March 2024.
"As a result of the work of the UK, Sweden and other partners, we have confirmed that a deadly toxin was found in Alexei Navalny's body," Ms Cooper told journalists,
"And that toxin has been identified as a toxin that is found in Ecuadorian Dart Frogs."
She said: "Only the Russian government had the means, the motive and the opportunity to use that toxin against Alexei Navalny in prison and that is why we are here today to shine a spotlight on the Kremlin's barbaric attempt to silence Alexei Navalny's voice.
"To show that the Russian government has contempt for its citizens and the willingness to use this deadly toxin."
The top British diplomat also quoted Mr Navalny himself about the need to hold the Russian government to account.
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"He said: 'We must do what they fear, tell the truth, spread the truth that is the most powerful weapon'."
Ms Navalnaya shared details about the effect of being poisoned dart frog toxin.
"My husband, Alexei Navalny, was poisoned with epibatidine - a neurotoxin, one of the deadliest poisons on earth," she said in a post of social media.
"In nature, this poison can be found on the skin of the Ecuadorian dart frog. It causes paralysis, respiratory arrest, and a painful death."
Sweden's foreign minister, Maria Malmer Stenergard, explained why it was necessary to understand the true cause of Mr Navalny's death.
"This is extremely important in order to be able to hold Russia accountable for what they have been doing and to continue to shine a light on their continuous lies," he said.
"We will now move forward with this information to the OPCW... This is yet another way to increase the pressure on Russia."
The Kremlin has yet to respond to the allegation.
President Putin's government has already been accused of previously trying to kill Mr Navalny in 2020 using a Novichok nerve agent - the same chemical weapon that Russia is believed to have deployed on the streets of Salisbury in a failed attempt to kill a former Russian double agent, Sergei Skripal, and his daughter in 2018.
Mr Navalny survived the original poisoning and recuperated in Germany before returning home to Russia, where he was arrested and jailed.
"I can imagine that these kinds of poisons are interesting [to Russia] because they are quite difficult to detect," says Dr Eric Franssen, doctor in clinical toxicology and pharmacology at the Department of Clinical Pharmacy, Onze Lieve Vrouwe Gasthuis hospital in the Netherlands.
Dr Franssen said: "Because in hospitals, we will not detect them in a routine toxicology screen. So perhaps this is a way to disguise intoxications."
The UK and its allies say the toxin was derived from an Ecuadorian dart frog. These are produced naturally to ward off predators.
Sky News understands it is likely the toxin was manufactured in a laboratory rather than actually taken from the frogs. In humans, the effects are deadly.
Dr Franssen added: "We do not know very much of these toxins in humans because you cannot experiment, of course, with this kind of toxins in real life, only in animals.
"But there have been reports that people may die after 10 to 20 minutes of intake of this when you have a certain amount of these toxins."
"This can be done by ingestion or injection in the bloodstream. I suppose then it has a direct effect on the muscles and the heart.
"And it paralyses the muscles, also your respiratory muscles. And then you become very low in your oxygen in blood. And also by cardiac failure, your brain does not receive any oxygen anymore and this may cause death."
"Nausea and vomiting are side effects of these kinds of toxins."
Dr Franssen's description tallies with photos, published by Navalny's foundation, of the prison cell where he was taken ill: vomit is clearly visible.
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Navalny was a political dissident who challenged Putin. He survived being poisoned with the Novichok nerve agent.
When he returned to Russia in 2021, he was imprisoned and convicted on trumped-up charges.
In December 2023 Navalny was transferred from a prison outside Moscow to a penal colony in Russia's Far North, in Kharp: the Polar Wolf colony, known for its brutal conditions, capable of holding around 1,000 prisoners.
Despite those conditions, Navalny seemed in good health - and good spirits - in a court appearance on 15 February, 2025, before his death.
Navalny died the next day.
The initial Russian explanation was that he lost consciousness after a walk.
His body was only returned to his family eight days after his death.
And the official state post mortem concluded the official cause of death was high blood pressure brought about by a chronic abnormal heart rhythm.
In September last year, Navany's widow, Yulia Navalniya, made a dramatic revelation. She posted on social media: "In February 2024 we were able to obtain samples of Alexei's biological material and securely smuggle them abroad. Labs in at least two countries examined these samples independently of each other and these labs in 2 diff countries reached the same conclusion Alexei was killed, more specifically he was poisoned."
Dr Franssen said this sort of testing was feasible.
He said: "Normally, when people die, you may have autopsies, then you cool the body because you do not want that the toxins degrade in the human body after death [in order to] detect them. So it may be possible to measure toxin levels in tissue.
"That can be the lung, that can be the heart, it can be blood and urine also. But it can also be possible to detect not the native drugs but also the metabolites of these toxins. So these can stay longer in the body."
That lab work has now been revealed by Britain and its allies.
Russian authorities insist Navalny's death was natural.
Sky News has learnt that the board of BrewDog has appointed AlixPartners, the restructuring experts, to field interest from potential bidders.
Sources said that AlixPartners had begun sounding out prospective suitors in the last few days, with a quickfire deadline for indicative offers understood to have been set.
A deal could see many of BrewDog's roughly 220,000 individual shareholders, who became investors through its 'Equity for Punks' scheme, left with little return from their average outlay of about £400.
Others, however, made a significant return on their investment by selling their stock in previous funding rounds.
The company raised about £75m overall from the sale of shares to customers, offering perks such as discounts and early access to new craft beers.
Its first such crowdfunding took place in 2009, with the most recent in 2021.
The sale process comes amid tough conditions for independent brewers, with the group behind Black Sheep recently being sold through a pre-pack administration.
Founded in 2007, BrewDog earned a reputation for innovation, with founders James Watt and Martin Dickie frequently courting controversy with the brand's marketing campaigns.
Mr Watt, who remains one of BrewDog's biggest shareholders, is now reportedly considering a bid to buy the company back, with sources indicating that he is canvassing support from financial backers.
One insider said the company would now command a price tag of far less than a once-mooted valuation of £2bn, which had stoked hopes among investors of an eventual stock market listing.
In 2017, TSG Consumer Partners took a 21% stake in BrewDog in a deal which implied a unicorn valuation of at least $1bn.
Since then, however, BrewDog has continued to face mounting losses, closing a string of bars and axing chunks of its workforce.
Last year, the company lost £37m on turnover of £357m.
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BrewDog trades from 72 bars globally, including in London and Las Vegas.
It employs roughly 1,400 people, and boasts a 4% share of the UK off-trade grocery market by value.
The company produces five of the top eight UK craft beer brands, including Hazy Jane, Wingman and Lost.
Its four breweries - at Ellon in Scotland, as well as sites in the US, Australia and Germany - could be sold separately from the rest of the group, depending on the progress of the AlixPartners-run process.
Five years ago, BrewDog was rocked by allegations made by dozens of former employees that it operated "a culture of fear".
Mr Watt stepped down as chief executive in 2024, and one source said this weekend that BrewDog's appearance in recent lists of reputable employers underscored a transformation in its culture.
"As with many businesses operating in a challenging economic climate and facing sustained macro headwinds, we regularly review our options with a focus on the long-term strength and sustainability of the company.
"Following a year of decisive action in 2025, which saw a focus on costs and operating efficiencies, we have appointed AlixPartners to support a structured and competitive process to evaluate the next phase of investment for the business.
"This is a deliberate and disciplined step with a focus on strengthening the long-term future of the BrewDog brand and its operations."
The company added that BrewDog "remains a global pioneer in craft beer: a world-class consumer brand, the No.1 independent brewer in the UK and with a highly engaged global community".
"We believe that this combination will attract substantial interest, though no final decisions have been made."
"Our breweries, bars, and venues continue to operate as normal.
"We will not comment on any further speculation."
Speaking at the Munich Security Conference on Saturday, the prime minister announced the move would be a demonstration of Britain's commitment to its allies.
He said: "I can announce today that the UK will deploy our carrier strike group to the North Atlantic and the High North this year, led by HMS Prince of Wales, operating alongside the US, Canada and other NATO allies in a powerful show of our commitment to Euro-Atlantic security."
It comes following US President Donald Trump's assertion earlier this year that he wants to take over Greenland.
Mr Trump has claimed he does not believe Denmark, of which Greenland is an autonomous territory, does enough to defend it from threats from Russia and China.
The US later walked back from the threats - but the geopolitical climate still looks increasingly precarious.
Russia is continuing to battle it out in Ukraine - a move the prime minister claimed was a "huge strategic blunder".
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And he pledged his commitment to Article 5, one of NATO's founding principles.
It spells out that an attack on one member is an attack on all.
Sir Keir said: "So I say to all NATO members, our commitment to Article 5 is as profound now as ever, and be in no doubt, if called on, the UK would come to your aid today."
If the article were to be triggered, it could mean thousands of Brits sent to the frontline.
But Sir Keir said we "must be ready to fight" and do whatever "it takes to protect our people, our values and our way of life".
"As Europe, we must stand on our own two feet," he added.
"That means being bolder, it means putting away petty politics and short-term concerns. It means acting together to build a stronger Europe and a more European NATO."
Maliki Sharma was taken to hospital and pronounced dead on January 12 after he was found with multiple stab wounds.
The Metropolitan Police said they were called to Victoria Road, Feltham, west London, at around 5pm following reports of a stabbing.
His family are being supported by specialist officers, police said.
The Met said a 16-year-old boy from Hounslow was charged with murder on Friday and appeared at Willesden Magistrates' Court on Saturday.
The boy, who cannot be named because of his age, has been remanded into custody and will next appear at the Old Bailey on Tuesday.
Two other 16-year-old boys, also from Hounslow, were charged with conspiracy to commit grievous bodily harm.
They appeared at Wimbledon Magistrates' Court and Uxbridge Magistrates' Court on Saturday.
Both have been remanded into youth detention and will appear at the Old Bailey on Tuesday.
Rhys Huskinson, 19, of North Road, Feltham was charged on Wednesday with affray and possession of an offensive weapon in connection with the investigation.
Police said that following further inquiries, the charges have been amended to attempted grievous bodily harm.
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Detective Chief Inspector Allam Bhangoo, who is leading the investigation, said: "My thoughts remain with Maliki's friends and family during this unimaginably difficult time.
"We understand that this incident has been distressing for the local community, and residents can expect to see an increased police presence over the coming days."
Huskinson has been remanded in custody to next appear at Isleworth Crown Court on March 11.