Prime Minister David Cameron has said he believes British warplanes, which have been bombing Islamic State targets in Iraq for more than a year, should also be tackling the group in Syria rather than 'sub-contract' UK security to other countries.

Keen to avoid a repeat of a damaging 2013 parliamentary defeat over plans to bomb the forces of Syrian President Bashar al-Assad, Cameron had made it clear he would not bring a vote to parliament if he did not think he could win it.

He is now all but assured of winning parliamentary backing after Corbyn said he would allow his lawmakers to vote according to their conscience -- breaking with a tradition for leaders to instruct lawmakers how to vote on big decisions.

Media reports say about 50 Labour lawmakers will back action when the vote takes place late on Wednesday, after 10-1/2 hours of debate in parliament. Air strikes could begin within days. When parliament approved strikes on Iraq, the bombing began four days later.

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