The legalisation of gay marriage is poised to be approved by MPs today after surviving a Tory backbench bid to derail it, lead by a Sussex MP.

East Worthing and Shoreham MP Tim Loughton yesterday saw a cut in support for his amendment to allow straight couples to take up civil partnerships.

It meant an embarrassing reverse was avoided after the Government accepted Labour calls for an immediate review of civil partnerships instead.

But if it clears its final stages in the Commons today, the Marriage (Same Sex Couples) Bill still faces stiff resistance in the House of Lords.

Conservative Mr Loughton, an education minister until last September's reshuffle, accused ministers of doing a "grubby deal" with Labour to see off his amendment and said the battle would continue in the upper chamber.

And the Conservative leadership remains under fire from many senior party members vehemently opposed to the measure.

One councillor last night accused ministers of showing "clear contempt for the deeply-held views of Conservative supporters" and fuelling an exodus to the UK Independence Party.

But Culture Secretary Maria Miller defended the Government's tactics insisting there was "overwhelming support" for the change, including among significant numbers in her own party.

Peter Tatchell, co-ordinator of the Equal Love movement, which has campaigned for same-sex civil marriages and opposite-sex civil partnerships, said he hoped the defeat of the amendment on legalising heterosexual civil partnerships would be "only a temporary setback for equality".

He said: "The government never wanted heterosexual civil partnerships and has used scare-mongering tactics to frighten MPs into voting to maintain discrimination against straight couples.

"There is no need for a review, since more than 60% of the public support civil partnerships for everyone, gay and straight.

"Maria Miller could have easily supported an amendment to legalise heterosexual civil partnerships on the condition that their implementation would be delayed for a year until the details were worked out. She chose not to do so."

He added: "Heterosexual couples have a right to a civil partnership. Banning them is wrong.

"This issue is all about equality. Just as gay couples should be allowed to marry, straight couples should be permitted to join together in a civil partnership. There should be no discrimination in civil marriage and civil partnership law."

But the Coalition for Marriage, which opposes the redefinition of marriage, said the Government had allowed Labour to "attach a £4bn ticking time bomb set to explode after a brief consultation on civil partnerships".

Campaign director Colin Hart said: "It's yet another twist that wasn't in anyone's manifesto. Civil partnerships are cracked open to allow in heterosexual couples.

"The Government has lost control, marriage has been left in limbo, it's a complete dog's breakfast, and the Bill should be dropped before more damage is done."