Updated 15.45

British Prime Minister David Cameron's described a new Islamic State video threatening the UK as "desperate stuff" from a group that's "losing ground".

The film features a masked man with a British accent making threats to the Prime Minister.

Intelligence agencies are now examining the footage to try to work out the identity of the militant.

Cameron says despite the 'despicable' actions of IS, they won't terrorise the UK:

Security and intelligence officials are still examining the video which purports to show a masked Islamic State militant with a British accent threatening the UK and calling Prime Minister David Cameron an "imbecile".

The video, which has not been independently verified, shows five men of Middle Eastern appearance wearing orange jumpsuits - who IS said were British spies - being shot dead.

A child aged about five, who also apparently spoke with a UK accent, also appears in the video wearing an IS headband and a camouflage outfit.

Authorities are working to identify the masked man, who is also pictured wearing combat fatigues and is heard describing the UK Prime Minister as a "slave of the White House".

The man said: "One would have thought you would have learnt the lessons from your pathetic master in Washington and his failed campaign against Islamic State".

"But it seems that you, like your predecessors Blair and Brown, are just as arrogant and foolish. In fact David you are more of an imbecile".

He added: "You will lose this war as you lost in Iraq and Afghanistan".

He is also seen waving a gun as he mocks Britain's Prime Minister.

In the 10-minute film, the five detainees apparently confessed to the allegations that IS accused them of.

Reporter Enda Brady said: "The prisoners are obviously speaking under extraordinary duress. Some of the men say they are from the town of Raqqa which has become the de facto capital of IS in Syria".

"And they apparently confess to the crime of spying for the UK government. That is what Islamic State accuse the men of. They are all then killed".

The video has not been independently verified.

In a statement, the UK Foreign Office said: "We are aware of the video and are examining its content".

The RAF has carried out at least 10 airstrikes against Islamic State in Syria since MPs backed the use of military force early in December.

UK warplanes have also been bombing IS targets in Iraq for a number of months.

The video emerged days after Mr Cameron promised to crack down on IS sympathisers, stressing in a New Year's message that all Britons should have "loyalty" to their country.

The footage also came in the wake of recent losses by IS, including in the Iraqi city of Ramadi which the army claimed to have captured from the militants.

The jihadist group has lost 40% of areas it once controlled in Iraq, according to US President Barack Obama.

And it has also lost a large amount of territory in Syria to Kurdish fighters.

Last November, British IS militant Mohammed Emwazi, known as "Jihadi John", who appeared in a number of beheading videos, was believed to have been killed in a drone strike in Syria.

A number of other UK jihadists have also been targeted and killed or are being hunted by authorities.

Jonathan Russell is from the British counter-extremism think tank the Quilliam Foundation.