Two British men were killed after being targeted in a drone strike, carried out by the Royal Air Force.
Reyaad Khan, 21, from Cardiff and Ruhul Amin from Aberdeen were both members of the Islamic State militant group and were hit on August 21 in an “act of self-defence” according to British Prime Minister David Cameron.
Mr Cameron accused Mr Khan of plotting “barbaric” attacks and said the drone strike was lawful and necessary.
The prime minister added the targeting of the men was approved by the attorney general.
In his statement to the Commons, Mr Cameron said: "My first duty as prime minister is to keep the British people safe.
"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."
However Labour leader Harriet Harman questioned the legality of the attack and called for “independent scrutiny”.
"Why didn't the Attorney General authorise this specific action rather than merely 'confirming there was a legal basis for it'?" she said.
"What was it about this individual and his actions that singled him out from all that has gone before?
"Did he represent an ongoing threat or was the threat based on a specific act he was plotting?"
Former Attorney General Dominic Grieve said tthe killing "would be justifiable if the factual basis is there", but that it is possible that the decision taken by the government could be "legally reviewed or challenged", the BBC reported.