A Royal Air Force (RAF) drone last month killed a British jihadist in Syria who was planning attacks on Britain, Prime Minister David Cameron said on Monday.
He described the operation as a first, as it involved an attack in a country where Britain was not legally at war, but said it was justified because it prevented terrorist bloodshed.
"Reyaad Khan was killed in a precision air strike carried out on August 21 by an RAF remotely piloted aircraft while he was traveling in a vehicle in the area of Raqqah," Cameron told parliament.
Two other Islamic State jihadists, including Briton Ruhul Amin, were also killed but no civilians were harmed, the prime minister said.
Cameron said the strike was "an act of self-defense" since Khan had been planning "barbaric" attacks in Britain against high-profile commemorations over the summer.
He did not give further details.
The strike had been "entirely lawful" and carried out following consultation with the attorney general, he added.
"There was a terrorist directing murder on our streets and no other means to stop them," he said.
"Is this the first time in modern times that a British asset has been used to conduct a strike in a country where we are not involved in a war? The answer to that is yes," he added.
Britain used drone strikes during the war in Afghanistan, and is also using them against IS militants in Iraq after joining in the U.S.-led intervention launched last year.
Khan, a 21-year-old from Cardiff, Wales, went by the nom-de-guerre Abu Dujana Britani. He had written on Twitter how he was being prepared to become a martyr and boasted of executing prisoners.
He left for Syria in 2013 with medical student Nasser Muthana, prompting a desperate plea for his return from his mother Rukia.
"Please come back before it is too late. You are my only one son," she said.
Bangladeshi-born Amin, 26, grew up in Aberdeen, Scotland, before moving to Leicester in central England with his family.
Posing with Kalashnikovs, Khan and Amin appeared in an Isis recruitment video in 2014 after traveling to Syria.
A third Briton, Junaid Hussain, a computer hacker described as a key IS operative, also died in a separate U.S. airstrike, Cameron confirmed.
- Attack a 'big departure' -
The prime minister stressed that the British strike was a targeted attack against terrorism, but said he also supported Britain extending its anti-IS bombing campaign to Syria as well as Iraq.
Cameron's government was defeated on taking military action in Syria in 2013 in one of the most damaging foreign policy blows to his previous coalition government.
Defense think tank Royal United Services Institute (RUSI) called the strike against Khan a "big departure" for Cameron.
"The point is not so much that this man was British, but that he was targeted in an area that the UK does not currently regard, legally, as an operational theater of war for UK forces," said RUSI director general Michael Clarke.
"The government was expected to introduce a motion into parliament to authorize the extension of air operations into Syria within a couple of weeks.
"It now looks as if it has decided to create a momentum to action that might be unstoppable," he added.
Jeremy Corbyn, the favorite to win the leadership of the main opposition Labor Party later this week and founder of the Stop the War group, repeated his opposition to air strikes on Monday.
"My view is that it would create more problems than it would solve," he said.