The Government has said it will consider whether to allow heterosexual couples to have a civil partnership rather than a marriage, at a cost of up to £4bn.
Equalities Minister Maria Miller hopes the promise of a review will buy off votes and give a smoother ride for the same-sex marriage Bill.
Allowing heterosexuals to have a civil partnership instead of a marriage would cost the country up to £4bn in public service pension rights alone.
An amendment allowing straight civil unions has been tabled to the same-sex marriage Bill, and the Government is keen to see the amendment fail.
The Government has offered an alternative amendment which would promise a review of civil partnerships five years after gay marriage is legalised.
The review will include a discussion of whether to scrap the whole civil partnership system.
Tory MP David Burrowes says the last minute revision is evidence that the Government hasn’t thought things through.
Redefining marriage has “profound consequences”, he said, which “require proper consideration and debate and this is not the way to do it”.
A Government spokesman said: “A proper review will allow us to look at the issues in a considered and thorough way, giving full consideration to the implications of any changes.”