The targeted killing of a suspected British terrorist by an RAF drone strike in Syria has sparked criticism of a "dangerous precedent" and warnings it will spark a legal challenge.
David Cameron revealed that jihadist Reyaad Khan, accused by the intelligence services of plotting "barbaric" attacks on "high-profile public commemorations" in the UK, was killed by a missile fired at his vehicle on August 21.
The unprecedented strike against a UK national - and the first British military action in modern times outside a war zone - was approved by the National Security Council but went ahead without the prior approval of Parliament.
Fellow British citizen Ruhul Amin was one of two other fighters for the self-styled Islamic State (IS) who died in the blast which came three days before a US drone killed another alleged British plotter, Junaid Hussein.
Events presided over by the Queen - including one marking the 70th anniversary of VE Day in June - were reported to have been among the targets, but Downing Street said confirming details could disrupt ongoing prosecutions.
The Prime Minister 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 recruiting jihadists and orchestrating a number of plots.
Attorney General Jeremy Wright had agreed there was a "clear legal basis", he told MPs.
"There was a terrorist directing murder on our streets and no other means to stop him."
Labour called for the legal advice to be published and Mr Wright's predecessor as the Government's chief law officer, Dominic Grieve, said he expected a challenge in the courts on human rights grounds.
Senior Conservative David Davis called for checks on what otherwise amounted to an "extra-judicial execution" - echoing concerns that have dogged the US administration which has long pursued a policy of targeted assassinations by drone.
The move was also seen by experts as part of a push towards extending the formal UK military action against IS into Syria, with David Cameron keen to secure Parliamentary support for an escalation.
At present, UK forces are restricted to operating in Iraq, but pressure to join US-led coalition air strikes on IS strongholds over the border has been increased after France declared it intends to do so.
Michael Clarke, director general of the Royal United Services Institute, said the Government appeared to have "decided to create a momentum to action that might be unstoppable".
And he warned Mr Cameron that he "may have to say more" about the legal justification for the significant shift in policy to satisfy critics.
Jeremy Corbyn, the frontrunner to be named the new Labour leader at the weekend, questioned the legal basis for the use of drones.
"Urgent consideration now needs to be given to the appropriate process by which attacks such as this one are sanctioned, on what evidence and on what basis of law," he said.
The election of Mr Corbyn as Opposition leader could make it harder for the Government to obtain parliamentary approval for military action in Syria - though ministers have hinted they could still attract sufficient Labour support.