The Prime Minister told Tory MPs not to side with Jeremy Corbyn and others opposed to the strikes
British Prime Minister David Cameron has repeatedly refused to apologise for branding opponents of airstrikes against Islamic State targets in Syria "terrorist sympathisers".
As he set out his case for launching military action in the House of Commons, he was asked by a number of opposition MPs to say sorry for the comment.
Mr Cameron caused controversy ahead of the vote when he told Conservative MPs not to side with Labour leader Jeremy Corbyn and "terrorist sympathisers" by opposing airstrikes.
When challenged about this in the Commons, Mr Cameron insisted no one was arguing about the need to tackle terrorism and there was "honour" in voting either for or against military action.
He said: "I respect people have come to a different view from the Government than the one I will set out today and those who vote accordingly."
Mr Corbyn said the remark "demeans the office of the Prime Minister and undermines the seriousness of the deliberations we are having today".
Parliament will vote on extending airstrikes - currently restricted to Iraq - to IS targets in Syria after a one-day debate on Wednesday.
The Labour leader had wanted a two-day debate on the "highly complex situation" and accused the Prime Minister of "bulldozing" a matter of national security through the Commons "for political convenience".
He gave the go-ahead to a free vote to Labour MPs in the face of threatened resignations among the shadow cabinet if he tried to whip support against bombing raids.
Both Mr Corbyn and the shadow foreign secretary Hilary Benn will speak at the despatch box on Wednesday - even though they will vote differently.
Mr Corbyn said: "I agreed to a free vote that recognises there are differences of opinion within the party.
"The vast majority of party members are opposed to the war in Syria, the majority of Labour MPs are opposed to the war in Syria.
"But I have a spirit of democratic openness in which I agreed on a free vote so that every member has their responsibility tomorrow to make their decision on whether this country goes to war again or not."
He added: "I hope every MP will recognise tomorrow there's no hiding place on whipping or anything else, you've got to make up your own mind.
"But the important issue tomorrow isn't the process within the Labour Party, it's David Cameron trying to take this country into a bombing mission in Syria, claiming there is an army of 75,000 people about to support, whose existence is questionable shall we say and whose membership is certainly more interested either in fighting Assad or of doing deals with other jihadist forces.
"It seems to me we're stepping into something potentially rather dangerous and unknown but also when you bomb in Raqqa or anywhere else, any other place, you kill people."
The Cabinet has agreed on the Commons motion setting out the need for military action against the militant group.
The Liberal Democrats, which has eight MPs at Westminster, has said they will back airstrikes in Syria.
Forrmer party leader Nick Clegg told Sky News the situation had "changed very dramatically" in recent weeks following the Paris massacre.
The French had requested help and UN resolution had "demanded action" against IS, he said.
Mr Clegg added: "It's more of a judgement than a science."
However, a cross-party move has been made to block airstrikes with senior MPs from across the Commons tabling an amendment stating the PM had not made the case for war.
Members of the Foreign Affairs Committee have also said they are not convinced by David Cameron's response to their concerns about airstrikes.
Around a thousand demonstrators have gathered outside Parliament to take part in a protest organised by the Stop the War coalition against military action.
Speaking at Downing Street, Mr Cameron said: "I will be making the arguments and I hope as many Members of Parliament - across all parties - will support me as possible.
"That motion talks about ... the necessity of taking military action against ISIL (IS) in Syria as well as Iraq, but it is part of a broader strategy.
"It's about politics and diplomacy and humanitarian aid, all of which we need to bring to bear to bring peace to Syria but to make sure we protect our national interest of fighting against this appalling terrorist organisation."