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
David Cameron will be facing a hostile audience when he meets with leaders from the remaining 27 EU nations this week, writes James Mates.
Baroness Warsi warns reports of hate crimes are rising after a "divisive" EU campaign largely focusing on immigration spurred street abuse.
Labour has descended into crisis as several high-profile members of the shadow cabinet have quit in the past few hours. Here's who they are.