The Foreign Office's chief civil servant Sir Olly Robbins was sacked on Thursday night after Sir Keir and foreign secretary Yvette Cooper lost confidence in him over the scandal.
The prime minister was "not aware" there was a recommendation against giving Lord Mandelson security clearance to become US ambassador - which was overruled by the Foreign Office, according to a document produced by Number 10.
He was only told of the advice on Tuesday.
Sir Keir said he will "set out all the relevant facts in true transparency" to parliament on Monday.
The prime minister on Friday said he was "absolutely furious" and it was "staggering" that he had not been informed the vetting process was overruled to clear Lord Mandelson for the ambassador posting.
"I was not told that he had failed security vetting, no minister was told... Number 10 wasn't told, that is completely unacceptable," he said.
Criticising officials, the PM said: "It is totally unacceptable that the prime minister making an appointment is not told that security vetting has been failed."
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Sir Keir is not resigning or considering his position, No 10 also said on Friday.
Ms Cooper only found out Lord Mandelson failed security vetting on Thursday after being approached by journalists, Sky News understands.
Sir Olly has been invited to speak to the Foreign Affairs Committee as early as next Tuesday.
The opposition party leaders have all called for Sir Keir to resign over the latest revelations, with Kemi Badenoch, for the Conservatives, arguing the prime minister had misled parliament and the public.
SNP Westminster leader Stephen Flynn called on the prime minister to "do the decent thing and resign".
He suggested Sir Keir is "either incompetent, gullible or a liar".
The Foreign Office has been blamed for clearing the peer in January 2025 to start as US ambassador, despite him failing a secure vetting process.
The memo of a meeting between the country's most senior civil servants appeared to add credence to Sir Keir's defence that Foreign Office officials were responsible for clearing Lord Mandelson's appointment without his knowledge, and that he was informed of this on Tuesday evening.
But its publication by the government came as Sky News understands that two of those present at Tuesday's meeting, cabinet secretary Antonia Romeo, and cabinet office permanent secretary Catherine Little, had known for weeks about sensitive information linked to Lord Mandelson's vetting.
A Cabinet Office spokesperson said that the delay in telling the prime minister was due to legal checks on what information could be released.
They said: "Senior officials in the Cabinet Office did the right thing and took urgent steps to ensure they could update the prime minister.
"On receipt of the UK Security Vetting information, the permanent secretary of the Cabinet Office rapidly sought legal and other advice to be in a sound position to share the document, or the fact of it, with the prime minister. She updated the Cabinet Secretary who agreed with the approach.
"As soon as these checks were conducted, the Prime Minister was informed."
Lord Mandelson was sacked from his Washington role last September when more details emerged about his relationship with paedophile financier Jeffrey Epstein, who died in 2019.
Sir Keir was already facing heavy criticism over the decision to hand Lord Mandelson the position, despite it being known that his dealings with Epstein continued after the financier's conviction for child sex offences.
The Metropolitan Police has said the incident is not being treated as a terrorist incident and officers are keeping an open mind about the motive.
The force said officers were called at 10.31pm on Friday. The London Fire Brigade also attended the scene.
"A man approached a row of shops with a plastic bag containing what was later found to be three bottles containing fluid," the Met said.
He placed the bag next to the building and lit the items in the bag. The bottles failed to fully ignite and the man fled the scene.
The investigation is being led by Counter Terrorism Policing London, supported by officers from the North West Command Area.
No arrests have been made.
Commander Helen Flanagan, of Counter Terrorism Policing London, said: "At this stage last night's arson is not being linked to other incidents in the North West London area over the last week or last month's arson in Golders Green but Counter Terrorism officers are leading due to the similarities of each attack.
"I would ask that anyone with information or footage that could help our investigation gets in touch with police as soon as possible."
"We will be relentless in our pursuit of those responsible for this and other, similar, hateful acts against London's communities."
Hendon is just two miles away from Golders Green, where last month four vehicles operated by volunteer-led Jewish ambulance service Hatzola were destroyed.
Detective Chief Superintendent Luke Williams, who leads policing in the area, said that Friday's incident will only "add fears to the community".
"Since the incident in Golders Green last month we have spoken extensively to community leaders," he added.
"I understand and appreciate their concerns."
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Anyone going to a game at the MetLife Stadium in New Jersey from New York City's Penn Station will have to pay $150 (£111) for the 15-minute, nine-mile (14km) journey, US transport officials said.
That's nearly 12 times the usual $12.90 (£9.50) fare for the return trip from Manhattan to the venue in East Rutherford, which is home to both the NFL's New York Giants and New York Jets.
World Cup 2026 - everything you need to know
The World Cup's chief operating officer has warned that the price hikes will have a "chilling effect" that could tarnish the legacy of the competition.
With most fans unable or unwilling to shell out $225 (£166) for one of the limited number of parking spaces at the site, New Jersey officials expect around 40,000 fans will use mass transit for each match, including the final that's held at the same ground on 19 July.
England face Panama in their final group game on 27 June at the stadium, with Brazil, France and Germany also set to play there, possibly twice - should any of those sides reach the final.
The price hike came amid a row between state officials and FIFA over who should pay for fans to get to and from matches.
New Jersey Governor, Mikie Sherrill, said the uplift was necessary to ensure regular local commuters weren't stuck with a "tab for years to come" for hosting the first US World Cup since 1994.
NJ Transit officials said it would cost $62m (£46m) to transport fans to the stadium and, with only $14m (£10m) offset by grants, NJ transit president and chief executive Kris Kolluri denied it was "price-gouging", telling reporters on Friday they were "literally trying to recoup our costs".
The MetLife Stadium isn't the only venue where transport costs have been raised, as buses from various parts of Boston to the Gillette Stadium for tournament matches will cost $95 (£70), officials said this week.
Thousands of fans have already bought $80 (£59) round-trip train tickets for the 30-mile (48km) journey from the city to the arena in Foxborough, which normally cost $20 (£15) for New England Patriots games in the NFL.
Other World Cup host cities, including Los Angeles and Philadelphia, have said they'll be keeping transport fares unchanged, pointing to the $100m (£74m) or so provided in federal transport grants to host cities for additional buses and trains.
Ms Sherrill said on X on Wednesday that FIFA "should pay for the rides", to which world football's governing body replied that host cities agreed to offer free transportation for fans to all matches in deals signed in 2018.
Heimo Schirgi, chief operating officer for FIFA World Cup 2026, has hit back at Ms Sherrill's suggestion.
He called it "unprecedented" to demand FIFA absorb the costs of "arbitrarily set elevated prices".
"No other global event, concert or major sporting promoter has faced such a demand," he went on.
He warned that the pricing model will "have a chilling effect" that would "push fans toward alternative transportation".
"This increases concerns of congestion, late arrivals, and creates broader ripple effects that ultimately diminish the economic benefit and lasting legacy the entire region stands to gain from hosting the World Cup," he added.
Football writer Henry Winter also criticised the decision in an interview with Sky News.
"Welcome to America," he said. "It's dynamic pricing, we've seen it with tickets and we've seen it with travel.
"It's the land of opportunity, as they call it, and it's an opportunity for a lot of people there to make a lot of money out of visiting fans.
"Which is a pity because it's going to affect the atmosphere. It's going to affect the enjoyment of the supporters from all over the world.
"It is sad, it is wrong, and it is immoral."
The huge increase in the fare to MetLife also drew an objection from New York Governor Kathy Hochul, who said the neighbouring state's charge "sounds awfully high to me".
Speaking in Phoenix, the US president reminded the crowd how in February he had instructed his defence secretary Pete Hegseth to begin releasing files related to "alien and extra-terrestrial life".
At a Turning Point USA event on Friday, Mr Trump said: "This process is well under way, and we found many very interesting documents, I must say.
"The first releases will begin very, very soon."
Some US politicians have been pushing for a disclosure of government files related to unidentified aerial phenomena, or UAPs, after ex-president Barack Obama said in February "the odds were good there's life out there".
Mr Trump has not said he believes the phenomena are of alien origin.
The Pentagon did not immediately respond to a request for comment on the timeline for a release of documents, or what the release may contain, Sky's US partner NBC News reported.
Republican Representative Tim Burchett shared a clip online of Mr Trump's latest remarks alongside the caption: "Thank you for keeping your word to me."
But critics have suggested the president could be using the discussions around UFOs to distract from the controversy surrounding the Epstein files.
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Back in February, Republican representative Thomas Massie branded the move to release UFO documents "the ultimate weapon of mass distraction".
"The Epstein files aren't going away... even for aliens", he added.
The president, whose name appears in the files, has denied any wrongdoing in connection with Jeffrey Epstein's crimes.
The Pentagon said in November 2024 it received hundreds of reports on UAPs following a 2024 hearing, including 21 that were worthy of "further analysis" due to "anomalous characteristics and/or behaviours".
However, officials said they did not find evidence to substantiate life from another planet in any of the reports received.
Also, a Pentagon report from March 2024 found there was no credible evidence US authorities covered up findings of UFOs or UAPs that revealed extraterrestrial life, writing that "most sightings were the result of misidentification of ordinary objects and phenomena".
He was detained after a court in Ireland issued an arrest warrant
Dubai authorities said they received a judicial file and "immediately launched intensive search and surveillance operations, leading to the suspect's capture" within 48 hours.
They said the "Irish fugitive" was detained on 15 April over his "alleged role in an international organised crime network".
Dublin's high court previously named Kinahan as a senior figure in a crime gang involved in international drug trafficking operations and firearm offences.
In 2022, US authorities issued a reward of up to $5m for his arrest.
Kinahan was previously linked to the boxing world and co-founded a well-known management and promotions company - which later shut down.
In 2016, a bloody feud with the rival Hutch gang led to an attempt on his life when men disguised as armed police opened fire at a boxing weigh-in at a Dublin hotel.
Kinahan managed to escape but gang member David Byrne was killed, shot multiple times as he ran for the exit.
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Irish police confirmed on Friday that a man in his late 40s was taken into custody in line with the extradition agreement the two countries.
They said the arrest was a "matter for the authorities in the United Arab Emirates at this time".
"Today's arrest is another extremely important demonstration of the need for international law enforcement co-operation in tackling transnational organised crime," a statement added.