Prime Minister David Cameron is giving his MPs a free vote on the Marriage (Same Sex Couples) Bill on Tuesday, which will also allow civil partners to convert their partnership to a marriage and enable married people to change their legal gender without having to end their union.
The free vote on the bill's second reading on Tuesday will avoid a technical rebellion, though the high numbers expected to oppose or abstain from within his party will still prove an embarrassment for the Prime Minister.
Strong Liberal Democrat and Labour support means the Bill, however, will comfortably reach its next stage.
Culture minister Ed Vaizey insisted the vote would not tear the Tory party apart, and claimed internal divisions over the issue were "good-natured".
More top news
Police have revealed a pair of sandals they have had made similar to the pair Ben is thought to have been wearing when he disappeared
The hospital was last attacked just three days ago, something that UN Secretary General Ban Ki Moon denounced as a war crime.
Former UKIP leader Nigel Farage has flown to the US to help coach presidential candidate Donald Trump ahead of his next debate.