A prominent Conservative has broken ranks with his allies on the "religious right" by declaring his support for the Government's controversial plans to legalise gay marriage.

In an interview with The Independent, Tim Montgomerie dismissed criticism that extending equal rights to gays and lesbians would weaken marriage. He said Tories and church leaders should support gay marriage because it would save the institution, not destroy it. "Marriage is probably the most important Conservative institution and excluding people from it is therefore excluding people from Conservativism to a significant extent," he said.

Mr Montgomerie, the editor of the ConservativeHome website for Tory activists, criticised Dr John Sentamu, the Archbishop of York, who last week warned David Cameron he would be acting like a "dictator" if he allowed same-sex couples to wed. "He is a very good man and I respect him," Mr Montgomerie said. "But on issues like this people have a responsibility to use language carefully."

Mr Montgomerie's move is significant because in 1990 he founded the influential Conservative Christian Fellowship (CCF) with David Burrowes, a Tory MP who now leads the campaign against gay marriage. He admitted that many friends and natural allies in the organisation were "upset, very perplexed" and felt "real disappointment" at his decision. He will not resign from CCF. When he told Mr Burrowes about his view, they had a "friendly discussion" but "we did not persuade each other of our position".

Mr Montgomerie's declaration is a coup for gay rights campaigners and a setback to the Tory MPs plotting to defeat Government proposals to be published next month. Insiders predicted yesterday that 50 Tories might vote against the move and another 50 could abstain. A ConHome poll of Tory members in December found they opposed the Government's plans by 55 per to 36 per cent.

Ben Summerskill, the chief executive of Stonewall, said: "We're delighted that, having heard the arguments, one of Britain's most influential evangelical Christians is now able fully to support marriage for gay people without compromising his faith in any way. Many people of faith are considerably more progressive than the religious leaders, such as Dr Sentamu, who often speak intemperately in their name."

Mr Montgomerie said: "Conservatives should want as many people as possible to live in institutions and social norms that promote stability, faithfulness and compassion. Marriage is an institution at the centre of society. It is because I value it so much that I want it to be extended. This is not about equal rights. It is about extending an incredibly important social institution." Admitting the Tories previously had a "terrible" record on gay rights, he said: "To put it kindly, it has dragged its feet for a long time." He pointed out that Mr Cameron had consistently favoured gay rights since becoming party leader.

Mr Montgomerie, who described himself as "not married and not gay", said: "The Conservative Party has never had any shortage of gay people in it. The best way to think about it is: what David Cameron is embarked upon is an incredibly important project – to make social conservatism fashionable again. Marriage is civilising, stabilising, a hugely important institution for bringing people together. But if marriage is fossilised and exclusive, that has only limited reach. His attempt to enlarge and modernise the institution should not be seen as a threat to marriage but as its saviour."

Admitting he had travelled a long way to reach his current position, he said: "I understand why those who value it so much want to protect it as it is. I have lot of sympathy with Christians and Conservatives who feel we tamper with it at our peril."