The deputy leader of the British Labour Party, Tom Watson, has urged the Prime Minister David Cameron to provide "more clarity" on his strategy for fighting Islamic State.
MPs will vote on extending airstrikes against IS targets to Syria after a one-day debate on Wednesday.
Labour leader Jeremy Corbyn granted his MPs a free vote on the issue despite his own anti-war stance.
That means David Cameron looks to have the "clear majority" he wanted before bringing Syrian airstrikes to the Commons for a second time.
But in a letter to the PM, Mr Watson indicated that Labour MPs' support for the strikes 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".
'Terrorist headquarters in Syria'
Earlier, Mr Cameron said: "I believe there is growing support across Parliament for the compelling case there is to answer the call from our allies to act against ISIL (IS) in Syria and Iraq".
"The headquarters of the terrorists is in Syria and it makes no sense to recognise this border in the action we take when ISIL themselves don't".
"It's in the national interest, it's the right thing to do. We'll be acting with our allies and we'll be careful and responsible as we do. In my view it's the right thing to do this to help keep our country safe".
Mr Corbyn had asked for a two-day debate on the "highly complex situation".
On why he refused this request, Mr Cameron said: "We will make sure that we have a very long and full debate on Wednesday and we will take the action necessary to make sure we have, in many ways, the equivalent number of questions we would often have across a two-day debate in one day".
But Mr Corbyn hit back, accusing Mr Cameron of "bulldozing" a matter of national security through the Commons "for political convenience".
British Foreign Secretary Philip Hammond said he was "confident" a majority could be secured.
Despite offering his MPs a free vote, Mr Corbyn will make clear that the party's official policy is to oppose military action.
