A British citizen believed to be plotting Islamic State terror attacks in the UK has been killed in an RAF drone strike in Syria, David Cameron has revealed.
The strike against Reyaad Khan was carried out without parliamentary authority on August 21 and was the first occasion in modern times that the UK has used military force in a country where it was not engaged in a war, the Prime Minister told the House of Commons.
Three days later, a second Briton believed to have been involved in plots against the UK, Junaid Hussain, was killed in a US air strike.
Mr Cameron said the targeting of Khan in the IS stronghold of Raqqah was justified on the grounds of "self-defence", as he and Hussain were actively involved in orchestrating a number of plots to attack "high-profile public commemorations" over the summer.
This year saw a number of major commemorations, notably a service attended by the Queen to mark the 70th anniversary of VJ Day, as well as Armed Forces Day in June and events to mark the 10th anniversary of the July 7 attacks on the London transport network. Downing Street declined to identify which events were targeted, citing the need to avoid disrupting ongoing prosecutions.
Two IS fighters - including British national Ruhul Amin - were also killed in the attack on the vehicle in which Khan was travelling, but no civilians died, said the Prime Minister.
Plans to target Khan and Hussain were approved by a meeting of senior members of the National Security Council, attended by the Prime Minister earlier in the summer, but the operation was authorised by Defence Secretary Michael Fallon.
News of the mission came amid expectations that Mr Cameron will soon ask Parliament to vote on extending UK air strikes against IS - also known as Isil or Isis - from Iraq into Syria, though the PM stressed that the killing was not part of coalition operations.
It emerged hours after President Francois Hollande announced that France will start reconnaissance flights over Syria on Tuesday with a view to launching attacks on IS militants, but ruled out sending troops on the ground.
The Prime Minister insisted that the strike was "entirely lawful" and was approved by Attorney General Jeremy Wright.
In a statement to the Commons, Mr Cameron said: "My first duty as Prime Minister is to keep the British people safe. That is what I will always do.
"There was a terrorist directing murder on our streets and no other means to stop him.
"This Government does not for one moment take these decisions lightly. But I am not prepared to stand here in the aftermath of a terrorist attack on our streets and have to explain to the House why I did not take the chance to prevent it when I could have done."
Interim Labour leader Harriet Harman called on the Government to publish Mr Wright's advice, while former attorney general Dominic Grieve predicted that Khan's killing would "probably lead to a legal challenge in due course".
Senior Conservative David Davis, a former shadow home secretary, told BBC Radio 4's PM there should be a formal check on such decisions - suggesting they otherwise amounted to an "extra-judicial execution".
Mr Cameron said intelligence agencies had identified a direct threat permitting the UK to respond under the "inherent right of self-defence" contained in the Charter of the United Nations.
Khan and Hussain were "involved in actively recruiting Isil sympathisers and seeking to orchestrate specific and barbaric attacks against the West, including directing a number of planned terrorist attacks right here in Britain, such as plots to attack high-profile public commemorations, including those taking place this summer", he said.
"We should be under no illusion. Their intention was the murder of British citizens. So on this occasion we ourselves took action."
Britain had no way of preventing Khan's planned attacks without taking direct action, said Mr Cameron, telling MPs: "I am clear that the action we took was entirely lawful."
He said the military "assessed the target location and chose the optimum time to minimise the risk of civilian casualties".
Mr Cameron said the Government reserved the right to take military action without Commons approval if there was "a critical British interest at stake or there were a need to act to prevent a humanitarian catastrophe".
Downing Street said any future decisions on whether to target IS militants believed to pose a threat to the UK would be taken on a case-by-case basis, and declined to say whether any other such strikes have been authorised.