A Catholic clergyman who described gay marriage as "madness" faced criticism on Sunday when he was accused of scaremongering and Downing Street reiterated the prime minister's personal support on the issue.

In an article for a Sunday newspaper, Cardinal Keith O'Brien accused the coalition of trying to "redefine reality" with its proposal for legalising gay marriage, which is due to go out for consultation later this month.

The letter reignited a sometimes passionate debate over the issue, which David Cameron made a personal crusade when he used his party speech two years ago to support gay marriage "because I am a Conservative".

A Downing St spokesman said there was no change in the government's pledge to legalise gay marriage, though he said Cameron was "relaxed" about church criticisms, which have also been voiced by the Archbishop of Canterbury.

"There inevitably will be some people in the church who disagree with us, but it's important for equality," he said.

Margot James, the first openly lesbian Conservative MP, criticised the "apocalyptic language" used by the cardinal and accused him of "scaremongering".

She told BBC 1's Andrew Marr Show: "I think it is a completely unacceptable way for a prelate to talk. I think the government is not trying to force Catholic churches to perform gay marriages at all. It is a purely civil matter."

She added: "I think this sort of scaremongering is what it is, it is just scaremongering."

Cardinal O'Brien wrote in the Sunday Telegraph: "Since all the legal rights of marriage are already available to homosexual couples, it is clear that this proposal is not about rights, but rather is an attempt to redefine marriage for the whole of society at the behest of a small minority of activists.

"Same-sex marriage would eliminate entirely in law the basic idea of a mother and a father for every child. It would create a society which deliberately chooses to deprive a child of either a mother or a father.

"Other dangers exist. If marriage can be redefined so that it no longer means a man and a woman but two men or two women, why stop there? Why not allow three men or a woman and two men to constitute a marriage, if they pledge their fidelity to one another?"

Earlier this week the Home Office defended the plans after Tory MP Peter Bone called them "completely nuts".

A Home Office spokeswoman said the government believed that "if a couple love each other" and want to commit to a life together they should "have the option of a civil marriage irrespective of their sexual orientation".

Equalities minister Lynne Featherstone is due to launch a consultation on the plans later this month.

Labour's deputy leader Harriet Harman said she hoped the comments would not end up "fuelling or legitimising prejudice." She told the Andrew Marr Show: "We have had prejudice, discrimination and homophobia for hundreds of years, that doesn't make it right.

"I don't want anybody to feel that this is a licence for whipping up prejudice."