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
Ireland's overwhelming support for gay marriage is a message to the world, UN secretary general Ban Ki-moon said.
The owner was arrested after two police officers fell ill from the fake cannabis, known as Mr Nice Guy, Israeli media reported.
Thomas Cook chief executive Harriet Green is reportedly due to receive a personal bonus of £10.5m.