Addressing the Commons on Monday afternoon to confirm Khan was one of two Britons killed, the Prime Minister said: “In recent weeks it has been reported that two ISIL fighters of British nationality who had been plotting attacks against the United Kingdom and other countries have been killed in airstrikes”.

The prime minister told Parliament the attorney general had been consulted and had approved the drone strike by the Royal Air Force.

Cameron said the danger from IS fighters was more serious than it had ever been before and the drone attack was the only “feasible means” to deal with the threat.

Hussain and Khan were “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”, Mr Cameron said.

Ms Harman said there needed to be independent scrutiny of what the Government had done.

Mr Cameron said the strike was not part of coalition action against IS in Syria.

Mr Cameron told the Commons: “We should be under no illusion”. Their intention was the murder of British citizens. Cameron said the government reserves the right to take future action without prior approval when there is a “critical” British interest at stake or when a “humanitarian catastrophe” is imminent and can be averted.

But he resisted calls from acting Labour leader Harriet Harman to publish the legal advice supporting the drone strike.

Khan had appeared in an ISIL recruitment video past year. Two others travelling with the man – including another Briton – were also killed. Cameron informed Parliament that there were no civilian casualties.

He added: “If there is a direct threat to the British people and we are able to stop it by taking immediate action then as Prime Minister I will always be prepared to take that action and that’s the case whether the threat is emanating from Libya, Syria or from anywhere else”.

Also known as Abdul Raqib Amin, he was born in Bangladesh and grew up in Aberdeen before reportedly moving with his family to Leicester.

Cameron said Britain had a “moral responsibility” to resettle the people and would accept more refugees under the UN’s schemes.

And Kat Craig, legal director of Reprieve’s Abuses in Counter-Terrorism team, said: “Make no mistake – what we are seeing is the failed U.S. model of secret strikes being copied wholesale by the British government”. We have no military on the ground to detain those preparing plots.

The prime minister told MPs that the strike against the 21 year old in Syria was an “act of self-defence”, and that he authorised the attack.