It happened in northern Iraq on Sunday.
"The service person's family have been informed and have requested a period of grace before further details are released," said the MoD.
Defence Secretary John Healey announced the death to the House of Commons on Monday.
"I know the thoughts of the house will be with the family and with the unit at this desperately sad time," said Mr Healey.
Some British personnel remain in the region as part of a coalition supporting Operation Shader, designed to prevent a resurgence of the Islamic State group.
The mission includes training local forces in areas such as weapons maintenance, counter-IED skills, and medical and engineering.
UK forces are also involved in intelligence and surveillance, as well as strike operations in support of partner forces.
In March, the government acknowledged a British drone had killed a "known Daesh terrorist" in Syria.
Sunday's incident follows the death of a Parachute Regiment soldier in a training accident in Ukraine in December.
Lance Corporal George Hooley, 28, was killed while watching the Ukrainian military test a new weapon away from the frontline, the MoD said.
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Another British man died in Ukraine last month.
Ayrton Redfearn, 23, from Devon, was killed in action in the eastern Donetsk region.
He had joined a specialist unit last year and his mother said on Facebook he was due to get an award for saving someone's life five days before his death.
It is not known if Mr Redfearn had any formal military experience before going to the country.
Some 24 people were arrested at the event, the Metropolitan Police said, with the force thanking "the vast majority of people" who celebrated "safely and responsibly".
Among those arrested were 10 people on suspicion of assault on police. One officer suffered a slash wound to the hand and another was hit on the head, both from objects thrown at them, the Met said.
Three people were detained on suspicion of sexual assault, while one person was arrested on suspicion of grievous bodily harm after an assault causing a head injury.
Three people were arrested over alleged drugs offences, one of whom was also held on suspicion of having a lock knife.
Police said officers dealt with "six stabbings on the parade footprint" later in the evening after most of the crowds had dispersed.
One man in his 20s was taken to hospital in a life-threatening condition but is now stable.
Damage was also caused to four police vans in Islington, north London.
Commander Stuart Bell, who led the Met's public order operation this weekend, said most of those attending behaved "safely and responsibly".
He pointed to "pockets of anti-social behaviour and incidents where officers needed to intervene, including assaults on their colleagues.
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"As the evening progressed and the majority of the crowds made their way home, there was sadly further violence, including gang-related incidents.
"Officers were swiftly on scene to each stabbing and investigations are ongoing."
Speaking on stage at the Tyne Theatre in Newcastle on Sunday, Keegan, 75, joked that he is "still alive", referring to a fake story that was published after his diagnosis claiming he had only days to live.
The evening was reportedly his first public outing since his cancer diagnosis was revealed in January.
He told the audience he is responding well to treatment after being assured "'we have a top doctor with this new way of fighting what you have got".
"He [the doctor] was a Liverpool supporter, so I went to meet him. I knew I wouldn't be walking alone, if you know what I mean."
Keegan is closely associated with the Premier League teams Liverpool and Newcastle, the latter he both played for and managed, and he is affectionately known as "King Kev" for his footballing exploits.
The striker won three First Division titles while a player at Liverpool, as well as an FA Cup, two Charity Shields, two UEFA Cups, and the European Cup.
As a manager, he guided Newcastle to promotion to the Premier League and to second place in the table in 1996. Newcastle would finish second again the following year, after his departure mid-season.
But he said, despite his popularity at United, he doesn't want a statue at St James' Park, alongside those of another former manager, Sir Bobby Robson, and iconic striker Alan Shearer.
He said: "You will have to wait until I die... My statue is the way people receive me."
But he is planning to return to the stadium next season for what will be his first visit since an ill-fated second spell as manager in 2008.
Keegan said he wants to wave to the crowd before kick-off, as "I want to say goodbye. I didn't get the chance when I left the club last time".
But he said the visit will only happen if current Magpies manager Eddie Howe supports it.
Keegan also had stints at Southampton, Scunthorpe, and for German side Hamburg during his playing career.
He won 63 England caps, scoring 21 goals, playing at the 1982 World Cup, and was also later captain of the national team.
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Keegan was twice named European Footballer of the Year while at Hamburg and returned to England to play for Southampton and Newcastle before retiring as a player in 1984.
He became England manager in 1999, but quit the following year after a disappointing Euro 2000 campaign.
His last managerial job was his fairytale return to Newcastle United in 2008 but he quickly fell out with then-owner Mike Ashley and eventually won a constructive dismissal lawsuit against the club.
Newcastle United went on to be relegated that season.
Michael and Anthony Stewart were convicted by a jury of killing Anthony Littler in East Finchley in 1984.
Mr Littler, 45, was found dead in an alleyway close to East Finchley tube station on 1 May 1984, with no one identified as responsible for his murder at the time.
Following a cold case review in 2022, a new investigation began and detectives built their case by speaking to neighbours, former partners, friends and employers of the main suspects.
Michael and Anthony, who were aged 15 and 18 respectively at the time of the murder, were convicted on Monday following a trial at the Old Bailey. They had denied any involvement.
Mr Littler suffered two fractures to the skull, one of which was associated with "catastrophic" brain injury.
Detective chief inspector Neil John, who led the investigation, said: "Anthony's life was suddenly cut short when he was killed in a brutal attack by two teenagers who we now know had a clear propensity for the most sickening kind of violence.
"They targeted Anthony because he was alone, defenceless and walking down a dark alley in which they knew no-one would see them carrying out their horrendous assault. They lay in wait for someone to cross their path and tragically for Anthony, he became their unsuspecting victim.
"We know Anthony's murder has continued to cause his family pain all these years later, and we are pleased that they now know who was responsible for his death.
"We also want to pay tribute to those who came forward to provide information on the events of that night, as well as giving vital evidence in court. Without them, this verdict would not have been possible.
"The Met will always review any new evidence that is brought to us, no matter the length of time that has passed. We will use all of the resources available to seek out the truth and pursue new opportunities to get justice for all victims who have been unlawfully killed."
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The American star has confirmed she will compete at the HSBC Championships at Queen's Club in west London next week as a wildcard entry into the doubles draw.
The 23-time Grand Slam champion last played on the WTA Tour in September 2022, when she retired from professional tennis following a third-round loss at the US Open.
Williams said: "Queen's Club feels like the perfect place to begin this next chapter.
"Grass has given me some of the most meaningful moments of my career, and I'm excited to be back competing on one of the sport's most iconic stages."
Williams, who played her first professional match in 1995, had fuelled rumours of a return after re-entering the sport's anti-doping pool last year.
The seven-time Wimbledon champion intensified those rumours even further by playfully dodging questions in an interview with NBC.
Serbian tennis star Novak Djokovic had suggested she could target Wimbledon for a comeback, while her former coach Rick Macci claimed she was going "all out" in training to prepare for a return.
Her sister, Venus Williams, who is 15 months older than Serena, is still playing and competed at the Australian Open in January as a wildcard.
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Following Serena's US Open defeat, she avoided using the word retirement and instead said she was "evolving away" from tennis.
Williams teased Monday's announcement with a social media post that contained a video with the caption: "Good news travels fast."
She gave birth to her second daughter in 2023 and had appeared absorbed with life away from the court as a mother and businesswoman.
But videos emerged on social media of Williams back in training on court.
Tournament director Laura Robson added: "Serena Williams is one of the greatest athletes the world has ever seen, and we're delighted that she will be making her return to tennis at the LTA's HSBC Championships."