The British Prime Minister is set to face more questions over the legality of the targeted killing of two Islamic State fighters from Britain.

Reyaad Khan, from Cardiff, and Ruhul Amin, from Aberdeen, were killed in the aerial raid which was ordered without Commons approval.

David Cameron said the attack was carried out with the approval of the UK Attorney General, but without parliamentary backing.

Two years ago, MPs voted against British military action in Syria.

When asked about the UK airstrike, former head of the British Army, General Lord Dannatt told Sky News that "by the letter of what parliament has authorised that may well not be right".

"I'm accepting that the British parliament has not authorised action from the air over Syria at the present time.

"For all I know British drones have been operating in the airspace but gathering intelligence - delivering explosive ordnance and attacking issues is another matter.

"And by the letter of what parliament has authorised, that may well not be right."

When it was suggested that drone operators had targeted and taken out British individuals in Syria without the endorsement of the Commons, he said: "Put it like that, then that is wrong."

A third Briton, Junaid Hussain, was killed in a US airstrike, on 24 August, the Prime Minister said.

Mr Cameron defended the RAF raid, telling the Commons it was "necessary and proportionate", adding it was "entirely lawful".

Speaking in the Commons on Monday, the PM claimed Khan and Hussain were actively involved in orchestrating a number of plots to attack "high-profile public commemorations" over the summer.

"We were exercising the UK's inherent right to self-defence," he said.

"The airstrike was the only feasible means of effectively disrupting the attacks planned.

"These were part of a series of actual and foiled attempts to attack the UK and our allies."

One of the planned attacks, uncovered in a Sky News investigation, was aimed at last month's VJ Day celebrations in central London.