The masked Islamic State militant who beheaded an American journalist last week. Photo: Reuters

British intelligence agencies are “very close” to identifying the apparently-English jihadist who beheaded an American journalist, the UK’s ambassador to the United States has said.

MI5 and MI6 have been piecing together intelligence and evidence from the horrific video footage of the brutal murder to track down the identity of the Islamic State (IS) killer who has been dubbed “jihadi John”.

Ambassador Sir Peter Westmacott told US news network CNN that Britain is on the brink of establishing who he is.

“We are very close to identifying who this guy is,” he told the State of the Union programme.

Foreign Office and Home Office officials refused to comment on the claims.

“We do not comment on security matters,” a Foreign Office spokesman said.

Downing Street said the government was “stepping up” efforts to defeat IS terrorists and announced that Lieutenant General Sir Simon Mayall has been appointed as security envoy to the Kurdistan region of Iraq.

Body armour, night vision goggles and other non-lethal equipment will be supplied to Kurdish forces in the next few days, a spokesman said.

“As the government’s security envoy to Iraq’s Kurdistan region, General Mayall will support Kurdish and wider Iraqi efforts to counter Isil and work with Iraq’s leaders as they establish a unity government,” he added.

“General Mayall’s extensive experience of the region means he will be able to draw on a broad range of existing relationships across Iraq, the region and with close allies.”

But the government is facing demands to take tougher action against British men heading out to Iraq and Syria to fight for IS.

Since these men are swearing allegiance to a hostile State, they should all forfeit their British citizenship

Former shadow home secretary David Davis dismissed suggestions that possible new laws, dubbed Asbos for terrorists, would prevent Britons fighting for IS and said they must instead be stopped from returning to the UK.

Lord Carey, former Archbishop of Canterbury, also called for Britons fighting for IS in Iraq and Syria to lose their passports.

Writing in the Mail on Sunday, Lord Carey said: “They should not have access to the privilege of travelling under a British passport . . . and they certainly should not be able to travel back with the barbaric and bloodthirsty skills they have gained.”

Davis said the government’s response to the crisis in Iraq had been “tentative, uncertain, almost limp”, and suggested the men heading overseas to fight were committing treason.

In an article for the Mail on Sunday, Davis wrote: “Since these young men are in effect swearing allegiance to a hostile state, they should all forfeit their British citizenship – not just those who are dual nationals.

“Since this is an incredibly serious penalty, it should be done only after a proper public trial carrying all the public seriousness and opprobrium of a murder trial, because in many cases that is what it would be.

“As the Home Secretary reiterated yesterday, lawyers would say you cannot render someone stateless. Perhaps, perhaps not. Whitehall lawyers have been wrong before. Democracies have a right to defend themselves.

“IS is claiming to be a state. They can issue these young men with Islamic State passports if they so wish. It is not our problem that they would have trouble getting into any civilised country with them.”

Foreign Secretary Philip Hammond told The Sunday Times the killing was “an utter betrayal” of everything Britain stands for. “It is horrifying to think that the perpet-rator of this heinous act could have been brought up in Britain.”