The decision to authorise the use of RAF drones to strike individuals in Syria plotting attacks on the UK was taken "some months ago", Downing Street has revealed.
A meeting of senior members of the National Security Council, chaired by David Cameron earlier this year, received advice from the Attorney General that such attacks would be legal on grounds of self-defence, said the Prime Minister's official spokeswoman.
But she declined to confirm or deny whether the meeting had drawn up a "kill list" of named individuals for targeting, saying only that decisions on future operations would be taken on a "case by case" basis.
The details emerged as Defence Secretary Michael Fallon indicated that more drone strikes could be launched within weeks to kill terrorists in Syria plotting to wreak carnage in Britain.
Mr Fallon - who gave the final approval for the operation which killed militants Reyaad Khan and Ruhul Amin in the Islamic State stronghold of Raqqah on August 21 - said the Government would "not hesitate" to order further strikes of this kind.
His comment came as Mr Cameron came under pressure from the Muslim community in Khan's hometown of Cardiff to explain what evidence he had that the 21-year-old posed a threat to lives in Britain.
Former local councillor and family friend Mohammad Islam said: "Everyone is stunned that the Prime Minister has ordered the killing of a British national without getting parliamentary approval.
"The legality of this is not something that rests easy with people - especially given the past assurances about there supposedly being weapons of mass destruction in Iraq. I think it's only fair that people will be sceptical."
Mr Cameron told the House of Commons on Monday that Khan and fellow British jihadi Junaid Hussain - killed by a US drone on August 24 - were planning "barbaric attacks against the West", including terror plots targeting "high profile public commemorations" this summer.
His spokeswoman told reporters that the threat from Khan was not restricted to public commemorations, several of which - including the VJ Day anniversary attended by the Queen - had taken place by the time of his death. He was regarded as continuing to pose an imminent threat even after these events passed without incident.
Asked if further individuals had been identified for targeted killings, the spokeswoman told reporters: "The Government remains absolutely committed to doing what is necessary to protect British people here on the streets of Britain."
Mr Fallon told BBC Radio 4's Today programme: "There are other terrorists involved in other plots that may come to fruition over the next few weeks and months and we wouldn't hesitate to take similar action again."
He refused to be drawn on the number of terrorists planning attacks against Britain but said it was more than three and revealed plots were also uncovered against Australia and the United States.
Mr Fallon said: "It's extremely dangerous because these are attacks that have been and are being planned against major public events on our streets. They are potentially attacks on members of our armed forces and on others, which would be extremely dangerous and would obviously involve the loss of life.
"Government has a duty, where it has information and the ability to prevent such attacks, government has a duty to deal with it."
He said the jihadis had been involved directly in plotting with people in Britain but would not confirm if any arrests had been made in the UK.
The Defence Secretary said the Government was prepared to take wider military action against IS, also known as Isil or Isis, but a fresh vote would need to be held in the Commons, which rejected plans for airstrikes in Syria in 2013.
"At some point the new parliament will have to rethink the absurdity of us being able to strike against Isil in Iraq but not being able to strike Isil's command and control centres in north-east Syria," Mr Fallon told Today.
"To get parliamentary approval we have to be absolutely sure that we would win the vote, that we would establish a sufficient majority for it."
MPs on the Commons Foreign Affairs Committee are expected to quiz former attorney general Dominic Grieve on the legality of the strike at a hearing looking at UK policy in Syria.
The committee's chair, Conservative Crispin Blunt, who voted against military action in Syria in 2013, told BBC Radio 4's World At One: "We need to understand whether the self-defence case stands up in this.
"It is going to be difficult for us to do it without access to the intelligence, which is why I would support a full reference to the Intelligence and Security Committee to look at the precise circumstances of this particular strike."