David Cameron repeatedly refused to apologise today for branding MPs opposed to launching airstrikes against Isis in Syria "terrorist sympathisers", as the Commons prepared to authorise military action.
Jeremy Corbyn, who is opposed to the government's plan to launch RAF raids, said Cameron's comment "demeans" the office of prime minister.
Cameron is expected to win Wednesday's evening's vote which will take place at 10pm following a ten-and-a-half-hour debate.
However he was thrown off his stride at the start of the debate, as opposition MPs heckled and demanded he apologise for his remark.
Last night at a private meeting of his party, Cameron urged Tory MPs to back airstrikes and not walk through the voting lobby with "Jeremy Corbyn and a bunch of terrorist sympathisers".
The comment was in stark contrast to the non-partisan tone he has previosuly attempted to strike in order to convince as many Labour MPs as possible to support RAF bombing.
In an attempt to repair the damage, Cameron told the Commons today "there is honour in voting for, there is honour in voting against".
"I respect that governments of all political colours have had to fight terrorism," he said. "I respect people who come to a different view from the government. I hope that provides some reassurance to members."
"I respect people who disagree," the prime minister added. "We are all discussing how to fight terrorism, not whether to fight terrorism."
However he pointedly refused to apologise despite having been asked to do so by seven MPs, including those who intend to vote with him, within the first half an hour of the debate.
Corbyn told Cameron the views of MPs opposed to military action "must be treated with the upmost seriousness and respect".
"The prime minister’s attempt to brand those who planned to vote against the government as terrorist sympathisers both demeans the office of the prime minister and I believe undermines the seriousness of deliberations we are having today."
Pausing his speech, Corbyn gestured at Cameron and said he was happy to give way if the prime minister wanted to say sorry.
Former SNP leader Alex Salmond, who is opposed to military strikes, attacked Cameron for the "deeply insulting remarks". He said he could not "identify a single terrorist sympathiser" on the list of MPs opposed to strikes.
Labour MP John Woodcock, who backs military action, demanded Cameron apologise. But he also highlighted the deep split within Labour over the vote. Many members of the shadow cabinet, including shadow foreign secretary Hilary Benn, are expected to vote in favour of airstrikes.
Woodcock said: "No-one on this side of the House will make any decision based on any such remarks, nor will we be threatened into not doing what we believe is the right thing - whether those threats come from online activists or our own Despatch Box.”
The debate within Labour over whether to bomb Isis in Syria has turned bitter, with pro-bombing MPs complaining about intimidation and abuse from anti-war activists.
Labour MP John Mann intervened on Corbyn as the Labour leader spoke to demand threats against pro-strike MPs stop. "There is no place whatsoever in the Labour Party for anybody who has been abusing those members of the Labour Party who choose to vote with the government on this resolution," he said.
Corbyn said "abuse has no part in responsible democratic politics".
Setting out the case for military action, Cameron said action in Syria was not the same as the Iraq War. "This is not 2003. we must not use past mistakes for indifference or inaction," he told MPs. "Inaction is a choice. I believe it is the wrong choice."
"I know that the long term solution in Syria must ultimately be a government that represents all of its people and work with us to defeat the evil organisation of Isil," he said.
"This threat is very real, the question is this - do we work with our allies to degrade and destroy this threat or do we sit back and wait for them to attack us?"
Corbyn said "14 years of disastrous wars in the wider Middle East" showed why the UK should not launch airstrikes. "The spectre of Iraq, Afghanistan and Libya looms over this debate. To oppose another reckless and half-baked intervention isn’t pacifism. It’s hard-headed common sense," he said.
"It’s wrong for us here in Westminster to see a problem, pass a motion and drop the bombs pretending we’re doing something to solve it. That’s what we did in Afghanistan, Iraq and Libya. Has terrorism increased or decreased as a result?"
Cameron is expected to win the vote with a majority of around 100. Somewhere between 30 to 70 Labour MPs are expected to ignore Corbyn's opposition to airstrikes and vote with the government.