Labour is demanding David Cameron grants more time to debate extending British air strikes into Syria and has asked the Prime Minsiter to give "more clarity" about his plans for defeating Isis.
The Government has scheduled an all-day debate on Wednesday, scrapping Prime Minister's Questions to allow more time for MPs to consider Mr Cameron's seven-point strategy.
But Tom Watson, deputy Labour leader, warned that although he agreed there was a "compelling case" to bomb Isis in Syria, there was still no "broad consensus" among MPs, with many on the Labour benches yet convinced.
Mr Watson challenged Mr Cameron's claim that there were 70,000 moderate Syrian fighters who could seize Isis-held territory following air strikes, a figure that was described as "magical" by the Tory chairman of the Defence Committee Julian Lewis.
Around 50 Labour MPs are expected to vote in favour of air strikes in Syria on Wednesday, but Mr Watson warned the Prime Minister that their support cannot be guaranteed.
In his letter demanding more time be allocated to the debate, he wrote: "While many colleagues agree with the compelling moral and legal case for action, a large section of Labour MPs remain unconvinced about two areas in particular and would welcome more clarity on these issues.
"It is incumbent upon you as the Prime Minister to listen and engage with colleagues and to answer the legitimate questions I have raised on their behalf.
"I do not believe you have given proper time to build consensus.
"As Jeremy Corbyn has made clear, parliament needs more time to make a considered decision on whether air strikes can take place.
"Only then can MPs from all parties confidently articulate that decision to their constituents and the British people."
David Cameron was given a significant boost in his bid to win parliamentary approval after Jeremy Corbyn failed to convince his Shadow Cabinet to oppose the air strikes.
Labour MPs will now be given a free vote, with Mr Corbynopposing military action but his Shadow Foreign Secretary Hilary Benn voting in favour.
Speaking on the Today programme, Mr Benn praised Mr Corbyn for backing down and granting Labour MPs a free vote and also suggested his stance could change, depending on the motion put forward to MPs on Wednesday.
"People of principle can reach different decisions about how to deal with a threat... people of conscience have reached different views about what the right thing to do is and those views are sincerely held and we should respect them and we have to listen to many voices, our constituents our party members, our colleagues in the House of Commons and of course the discussions that we have had in the Shadow Cabinet.
"I think it is to the great credit of Jeremy as a leader that he has recognised that there is a difference of view on this most important of questions. And we have decided - he has decided - that we are going to have a free vote.
"I have reached the conclusion, obviously yet to see the Government's motion, but I have reached the conclusion that we need to take this action because there is a clear and present threat from Isil/Daesh."
Earlier on Tuesday Ken Livingstone said he was opposed to bombing Isis in Syria because it would make them "more determined to kill us".
He warned that air strikes would also serve as a "good recruiting sergeant" for the Islamic terrorists.
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