Two British citizens who were fighting for Islamic State (IS) were killed in an RAF drone strike in Syria which was carried out without parliamentary approval, David Cameron has said.
A third militant was also killed in the August 21 strike on a car in IS stronghold Raqqa, in what the Prime Minister described as an "act of self defence" against the primary British target who was planning "specific and barbaric" terrorist attacks in his homeland.
No civilians were killed and the strike was not carried out as part of coalition bombing operations against IS, also known as Isis or Isil.
The strike was "entirely lawful" and was approved by the Attorney General, the PM insisted.
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 them.
"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.
"That is why I believe our approach is right and I commend this statement to the House."
Mr Cameron said in recent weeks it had been reported that two British IS fighters who had been plotting attacks in this country had been killed in airstrikes.
"Both Junaid Hussain and Reyaad Khan, were British nationals based in Syria who 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," the PM said.
"We should be under no illusion. Their intention was the murder of British citizens. So on this occasion we ourselves took action.
"Today I can inform the House that in an act of self-defence and after meticulous planning Reyaad Khan was killed in a precision air strike carried out on August 21 by an RAF remotely piloted aircraft while he was travelling in a vehicle in the area of Raqqah in Syria.
"In addition to Reyaad Khan who was the target of the strike, two IS associates were also killed, one of whom - Ruhul Amin, has been identified as a UK national.
"They were IS fighters and I can confirm there were no civilian casualties.
Mr Cameron went in: "We took this action because there was no alternative. In this area, there is no government we can work with.
"We have no military on the ground to detain those preparing plots. And there was nothing to suggest that Reyaad Khan would ever leave Syria or desist from his desire to murder us at home.
"So we had no way of preventing his planned attacks on our country without taking direct action."
Mr Cameron said: "I am clear that the action we took was entirely lawful. The Attorney General was consulted and was clear there would be a clear legal basis for action in international law.
"We were exercising the UK's inherent right to self defence.
"There was clear evidence of the individuals in question, planning and directing armed attacks against the UK.
"These were part of a series of actual and foiled attempts to attack the UK and our allies and in the prevailing circumstamces in Syria the air strike was the only feasible means of effectively disrupting the attacks planned by this individual.
"So it was necessary and proportionate for the individual self defence of the United Kingdom."
He said the military "assessed the target location and chose the optimum time to minimise the risk of civilian casulaities", he said.
Mr Cameron said the Government reserved the right to take military action without prior Commons approval if there was "a critical British interest at stake or there were a need to act to prevent a humanitarian catastrophe".
Action had been approved at a meeting of "the most senior members" of the National Security Council, he said, which was attended by the Attorney General.