A large proportion of the alleged 70,000 “moderate” Syrian opposition fighters claimed by David Cameron are actually radical Islamists, it has been claimed.

Louise Haigh, a Labour MP, recounted that the Government’s National Security Advisor had said in a briefing to MPs that more than 40 per cent of the fighters fell into this group.

Ms Haigh posted on Twitter after the briefing, which took place on Tuesday: “National Security Adviser confirms number of moderates on ground in Syria is 40,000 rest are much more radical Islamists.”

Other Labour MPs present, some of whom are supportive of extending bombing, questioned Ms Haigh’s account, however.

The number is important because the Government claims local fighters on the ground could support British airstrikes and achieve objectives without the deploymnet of Western groundtroops.

Stephen Doughty said: “He didn’t say that Louise – and dismay in room from all sides that you have tweeted that from the meeting.”

Ms Haigh replied that she had posted a “direct quote” of the National Security Advisor, that there was no suggestion the briefing was supposed to be confidential, and that she was happy to correct any mistake if one came to light.

In pictures: Syria air strikes (2014) Show all 20 1 /20 In pictures: Syria air strikes (2014) In pictures: Syria air strikes (2014) Syria Syrian citizens check a damaged house that targeted by the coalition airstrikes, in the village of Kfar Derian, a base for the al-Qaida-linked Nusra Front, a rival of the Isis group, between the northern province of Aleppo and Idlib In pictures: Syria air strikes (2014) Syria A Syrian boy (L) looking at a destroyed car that activists say was targeted by the coalition airstrikes, in the village of Kfar Derian, a base for the al-Qaida-linked Nusra Front, a rival of the Islamic State group, between the northern province of Aleppo and Idlib In pictures: Syria air strikes (2014) Syria Parts of a missile that activists say was fired by coalition airstrikes, in the village of Kfar Derian, a base for the al-Qaida-linked Nusra Front, a rival of the Isis group, between the northern province of Aleppo and Idlib In pictures: Syria air strikes (2014) Syria Tthe guided-missile destroyer USS Arleigh Burke (DDG 51) launching Tomahawk cruise missiles against Isis targets In pictures: Syria air strikes (2014) Syria The USS Arleigh Burke (DDG 51) launches a Tomahawk cruise missiles in the Red Sea, to conduct strike missions against Isis group targets in Syria In pictures: Syria air strikes (2014) Syria The guided-missile cruiser USS Philippine Sea (CG 58) launching a Tomahawk cruise missile against Isis targets in Syria, as seen from the aircraft carrier USS George H.W. Bush (CVN 77) in the Arabian Gulf In pictures: Syria air strikes (2014) Syria US navy sailors standing watch on the bridge while Tomahawk cruise missiles are launched against Isis targets in Syria, aboard the guided-missile cruiser USS Philippine Sea (CG 58), in the Arabian Gulf In pictures: Syria air strikes (2014) Syria An F/A-18C Hornet, attached to Strike Fighter Squadron (VFA) 87 prepares to launch from the flight deck of the aircraft carrier USS George H.W. Bush (CVN 77) in the Arabian Gulf, to conduct strike missions against Isis group targets AFP/Robert Burck In pictures: Syria air strikes (2014) Syria The US-led airstrikes in Syria against Isis targets in and around the city of Raqqa In pictures: Syria air strikes (2014) Syria A fighter from the Isis group holds a piece of what the IS is saying is a US drone that crashed into a communications tower in the Syrian city of Raqqa In pictures: Syria air strikes (2014) Syria Fighters from the Isis organisation pray at the spot where the jihadist group said a US drone crashed into a communications tower in the Syrian city of Raqqa In pictures: Syria air strikes (2014) Syria Fighters from the Isis group load a van with parts that they said was a US drone that crashed into a communications tower in the Syrian city of Raqqa In pictures: Syria air strikes (2014) Syria Fighters from the Isis group load a van with parts that they said was a US drone that crashed into a communications tower in Raqqa In pictures: Syria air strikes (2014) Syria Fighters from the Isis group gesture as they load a van with parts that they said was a US drone that crashed into a communications tower in Raqqa. A US-led coalition on carried out its first air strikes and missile attacks against jihadist positions in Syria, with Damascus saying it had been informed by Washington before the operation began In pictures: Syria air strikes (2014) Syria A Syrian man rides his bike past a communications tower that was destroyed after a US drone crashed into it, according to fighters with the Isis group, in the Syrian city of Raqqa In pictures: Syria air strikes (2014) Syria People inspect a shop damaged after what Isis militants say was a U.S. drone crashed into a communication station nearby in Raqqa In pictures: Syria air strikes (2014) Syria A man holds the remains of what Isis militants say was a U.S. drone which crashed in Raqqa In pictures: Syria air strikes (2014) Syria Resident gather in the back of a van the remains of what Isis militants say was a drone which crashed in Raqqa In pictures: Syria air strikes (2014) Syria A man inspects the remains of what Isismilitants say was a U.S. drone which crashed into a communication tower in Raqqa In pictures: Syria air strikes (2014) Syria A man inspects the remains of what Isis militants say was a U.S. drone which crashed in Raqqa

The Government is briefing opposition MPs on its supposed intelligence in Syria ahead of a Wednesday vote about whether British fighter jets should join in bombing in Syria.

The Prime Minister’s spokesperson said of Ms Haigh’s spokesperson: “That's a misrepresentation of the briefing from the national security adviser.”

Other experts and commentators have also rubbished the 70,000 moderate Syrian fighters figure, however.

Julian Lewis, the Conservative MP who chairs parliament’s Defence Select Committee, said he was “extremely surprised” at the claim, which he characterised as “magical”.

“If you look at the analysis of seasoned correspondents, including journalists in the field ... who have written extensively on the nature of the opposition in Syria, the figures don't come anything near approaching 70,000 - or even half that number,” he said.

“And we had recently admissions from the Americans that the number of moderate fighters that they had successfully managed to train and integrate into the theatre was down in minute numbers and indeed single figures in some cases.

“So where are these magical 70,000 people and why have they been unable to roll back ISIL Daesh?”

MPs will tomorrow vote on whether to extend British airstrikes in Iraq to Syria. The UK’s allies, the US, and France, are already bombing Isis in Syria.