Labour deputy leader Tom Watson has appealed to David Cameron for "more clarity" on his strategy for tackling Islamic State (IS) ahead of Wednesday's crucial vote on extending air strikes into Syria.
Mr Watson - one of number of senior Labour figures to have indicated their support for further military action - warned that there was still no "broad consensus" among MPs on bombing IS in its Syrian heartland.
In a letter to the Prime Minister, he said that while he believed there was a "compelling case" for action based on the direct threat from IS, also referred as Isil, Isis or Daesh, to the UK, many Labour MPs were still unconvinced.
In particular, he challenged Mr Cameron's claim that there were 70,000 moderate opposition fighters on the ground in Syria, saying the Prime Minister needed to explain where they were and how they could be brought into the fight against IS.
He also called on Mr Cameron to set out in more detail his wider strategy for a political transition in Syria to end the country's bloody four-year civil war.
His intervention comes after the Prime Minister announced on Monday night that the Commons would vote on Wednesday after Jeremy Corbyn was forced to agree to give Labour MPs a free vote in the face of a threatened shadow cabinet revolt.
It is thought that about 50 Labour MPs could be prepared to vote for military action - more than offsetting any Tory rebels - ensuring a comfortable majority for the Government.
But in his letter to Mr Cameron, Mr Watson indicated that their support should not be taken for granted.
"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," he wrote.
"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."