The PM said James Foley's killing was "deeply shocking" but warned it was "not a time for a knee-jerk reaction".

Cameron also said the government would "redouble" efforts to stop Britons travelling to fight in Iraq and Syria.

Foley, 40, had been missing since he was seized in Syria in 2012. The UK police try to confirm the militant's identity.

The extremist group, the so-called Islamic State of Iraq and the Levant (ISIL), posted footage of the killing online, which has since been verified by the White House.

The Metropolitan Police has warned that "viewing, downloading or disseminating" the video might be an offence under terrorism laws.

The prime minister asked the heads of the police, the intelligence agencies and the armed forces whether there was any power they now needed - the answer, I'm told, was no.

Cameron, who returned from holiday to chair emergency meetings with Home and Foreign Office officials, said the killer had not yet been identified but said it was "deeply shocking" that he appeared to be British.

"Let's be clear what this act is - it is an act of murder, and murder without any justification," he told BBC political editor Nick Robinson.

The PM said "far too many" British citizens had travelled to Syria and Iraq to take part in the fighting there and the government would "redouble all our efforts" to stop people going.

Cameron reiterated that the UK would not get involved in another Iraq war and there would be "no combat boots on the ground".

NTJ/NJF