A fired up Alex Salmond blasted David Cameron after the prime minister reportedly described Labour's frontbench as a "bunch of terrorist sympathisers". The Conservative leader apparently launched the attack against Jeremy Corbyn and Shadow Chancellor John McDonnell when he addressed the 1922 committee ahead of MPs voting on UK air strikes on Syria.

"You should not be walking through the lobbies with Jeremy Corbyn and a bunch of terrorist sympathisers," Cameron reportedly told the backbench Tories. But the remarks seemed to have backfired after MPs urged the prime minister to retract his comments.

Labour Barrow and Furness MP John Woodcock, who has publicly opposed Corbyn's anti-Trident position, challenged Cameron on the issue and his plea was followed up by former SNP leader Salmond, who reiterated that 110 MPs from six different parties had backed an amendment in a bid to block air strikes on Islamic State (Isis) in Syria.

"I've examined that list very carefully and I cannot identify a single terrorist sympathiser among that list. Will he now apologise for his deeply insulting remarks?" the Gordon MP said. Cameron failed to apologise for the comments but stressed there is "honour in any vote that honourable members make".

The exchange came after Corbyn decided to give Labour MPs a free vote on the issue after he faced considerable pressure from his shadow cabinet. Shadow Foreign Secretary Hilary Benn, for example, reportedly threatened to resign from his front-bench role in protest if the Labour leader whipped the parliamentary party.

Up to 50 Labour MPs are expected to defy Corbyn's anti-bombing stance and vote for air strikes on Syria. Meanwhile, the Liberal Democrats, the Democratic Unionist Party (DUP) and the Ulster Unionist Party (UUP) are planning to back Cameron when the MPs vote in the Commons' lobbies on the night of 2 December.