LONDON (Reuters) - Britain's parliament is likely to vote on Wednesday to approve air strikes against Islamic State militants in Syria after months of wrangling over whether enough opposition Labour lawmakers would back military action.

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" national security to other countries.

The election of veteran anti-war campaigner Jeremy Corbyn as Labour leader in September complicated his plans. Corbyn has accused the British leader of rushing to war and has appealed to Labour lawmakers who favor military action to think again.

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 now appears likely to win 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.

However, Cameron caused controversy and risked stiffening opposition among Labour ranks by reportedly urging his Conservative Party's lawmakers to back air strikes rather than side with "Jeremy Corbyn and a bunch of terrorist sympathizers".

Corbyn's spokesman was reported to have responded that this was a "contemptible slur". He did not immediately answer a request for comment from Reuters.

A spokeswoman for Cameron's Downing Street office did not offer an official comment.

Media reports prior to Cameron's remarks emerging said about 50 Labour lawmakers would 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.