A senior Conservative MP has challenged David Cameron over his "magical" claim that there are 70,000 moderate fighters in Syria who could seize Isis-held territory following a bombing raid on the Islamic extremists in the north of the country.

The claim has also become a sticking point for Labour, with Jeremy Corbyn expected to demand clarification over the 70,000 figure before deciding his party's position on air strikes.

Julian Lewis, chairman of the Defence Committee, said he was "extremely surprised" to hear the Prime Minister tell MPs that there were "about 70,000 Syrian opposition fighters on the ground who do not belong to extremist groups" when he set out the case for launching air strikes against Isis targets in Syria.

Mr Lewis said he was tabling a question in the House of Commons demanding Mr Cameron clarify the claim.

Following a Shadow Cabinet meeting to discuss Mr Cameron's seven-point plan for defeating Isis, which he set out in a 32-page document in response to a Foreign Affairs Committee report that expresed grave concerns over military action, senior Labour figures made clear that Mr Cameron would have to give more information about the number of forces on the ground in Syria.

The Prime Minister’s spokesman said the 70,000 figure was based on the “best intelligence and analysis we have”, adding: "The figure was provided to him by the Joint Intelligence Committee; they provided that intelligence and analysis independent of the Government.

"That's the best advice we've received; we've no reason to doubt it in anyway. He [the Prime Minister] obviously felt comfortable with the analysis provided to him."

Mr Lewis said intervention in Syria should not go ahead without the involvement with President Assad's army, saying: "Sometimes the best you can do is choose the lesser of two evils."

He told Sky News: "Where are these magical 70,000 people and if they are there fighting, how come they haven't been able to roll back Isil/Daesh? Is it that they're in the wrong place? Is it that they're fighting each other? Or is it that in reality they're not all that moderate and that there are a lot of jihadists among them?

"I think we really need to know about this so that we don't look back on this moment as having made a big mistake on the base of misleading information that was given not by the Prime Minister but to the Prime Minister.

"The Prime Minister has conceded that air strikes alone cannot be decisive in taking on Isil/Daesh, unless they are supported by credible ground forces. So the only missing element is the credible ground forces."

He added: "The reality is if you want to defeat Isil/Daesh, you need to have the regular Syrian army as part of the force that's going to do it and that's where the Prime Minister has a sticking point but he still can't bring himself to forge an alliance with the Russians and their client [President] Assad because, of course, they are... so unpleasant.

"But Churchil, who we are constantly told we must emulate, did precisely that with Stalin and the Bolsheviks in World War II.