Tories warn Cameron that gay marriage will split party in two

MPs describe move as 'barking mad' – but Boris Johnson among those to back proposals

David Cameron has been warned that he could tear the Conservative Party in two if he goes ahead with legislation that will permit churches to hold marriage ceremonies for gay people.

The Government is due to make an announcement this week that will give more detail of precisely what it is proposing, against a rising tide of outrage on the Tory back benches that the Prime Minister has been won over by supporters of gay marriage.

Yesterday, the Monmouth MP David Davies described the proposal as "barking mad" and accused of Mr Cameron of wanting the political kudos of being "carried shoulder high" by the gay rights pressure group, Stonewall. Peter Bone, MP for Wellingborough, accused the Prime Minister of having "no mandate" for a reform that will "split" the party.

But Mr Cameron will be cheered by the heavyweight Tories who spoke out on his side of the argument yesterday, including London's Mayor, Boris Johnson, and two cabinet ministers, Michael Gove and Patrick McLoughlin.

They were among 19 signatories of a letter to The Sunday Telegraph arguing that "marriage should be open to all, regardless of sexuality". Mr Johnson told Sky News: "It is perfectly obvious that the constituency is there for doing this – whack it through." The former Prime Minister Sir John Major also praised what he called "a courageous and genuine attempt to offer security and comfort to people who at present may be together, yet feel apart". He added: "We live in the 21st century and must move on: every couple should have the opportunity and the right to formalise their relationship."

The Government issued a consultation paper about gay marriage in March, which excluded the idea of ceremonies in churches or other religious buildings, but Mr Cameron has since declared he is in favour of allowing same-sex couples to marry in places of worship, with safeguards written into the law to protect the legal rights of churches which oppose the change.

Despite his promise to allow Tory MPs a free vote, the issue is threatening to cause lasting resentment. Up to 130 MPs have threatened to vote against it. Speaking on BBC Wales yesterday, Mr Davies warned: "We're going to lose a large number of very loyal activists who've gone out and campaigned for us over the years and who don't like this idea, so politically it's barking mad".

He added: "There is a political calculation, at some level, that this is going to be good and if we go ahead with it David Cameron's going to be carried shoulder high back into No 10 by Stonewall activists."

Mr Bone told Sky: "If this were a genuine free vote, which of course it isn't, I reckon most Conservative MPs would vote against." He added: "The Prime Minister is absolutely wrong on this. This 'cast-iron guarantee' he has given that no church will be forced to marry someone is obviously false, because the European courts will intervene.

"What will happen when a couple goes to a local church, same sex, saying they want to be married [and] that church turns them down? Off to a European court and, heigh-ho, all churches will be forced to.

"It was in no party manifesto, there is no mandate for the Prime Minister to do this; he is absolutely wrong to be doing it now, and he's splitting the Conservative Party when we don't need it to be split."