David Cameron is expected to launch British military action in Syria within days after Jeremy Corbyn in effect guaranteed that the prime minister could secure a Commons majority for war.
Mr Corbyn backed down on Monday on his previous plan to order his divided party to oppose extending RAF air strikes to Isis targets in Syria; instead Labour MPs will now be given a free vote.
The prime minister reacted quickly to Mr Corbyn's capitulation, announcing after he returned from the Paris climate summit that he would recommend to the cabinet on Tuesday that a one-day debate and vote on military intervention in Syria be held on Wednesday.
Mr Cameron's announcement reflects his confidence that he can now command a Commons majority. He had previously warned that defeat would deliver a "propaganda coup" to Isis.
Dozens of Labour MPs are expected to join Conservatives, Northern Ireland unionists and even Liberal Democrats — who opposed the 2003 Iraq war — in supporting Mr Cameron's call to arms.