The debate over gay marriage has escalated as secular and gay rights groups hit back at suggestions the sanctity of marriage could be undermined by legalising same sex unions, accusing the Catholic Church of "giving people permission for bigotry".

The backlash follows an article in the Daily Telegraph, which revealed the Archbishop of Westminster Vincent Nichols will intervene in the debate on gay marriage in a letter to be read in 2,500 churches this Sunday.

The Roman Catholic leader's letter warns extending marriage could compromise the “distinctive nature” of current partnerships.

In a letter, co-signed by the Archbishop of Southwark, they write: “Neither the Church nor the State has the power to change this fundamental understanding of marriage itself," adding heterosexual marriage is “at the foundation of our society."

But Terry Sanderson, head of the National Secular Society, said the law on gay marriage was "nothing to do with the Church" and "homophobia" lay at the heart of the latest comments.

"What the government is actually proposing is entirely secular in nature, they are only proposing changes to civil marriage," he told The Huffington Post UK.

"I don't think there's any doubt that for all their double talk about it being about religious freedom homophobia does lie at the heart of it. If you see the sort of language the pope uses about gay people it's not just a kind of 'we stick by our biblical morals' it's a kind of 'we are repulsed by this'.

"I think it's very sinister really that these men are using these kind of language. The Catholic Church is shrinking in Britain but in other nations where there's already a culture of homophobia, it gives a kind of permission to be brutal towards gay people.

"Absolutely the Church is responsible for increasing homophobia. They've got permission from the church for bigotry.

"The Catholic Church appears to have decided to create these conflicts with the state - there seems to be a kind of desire by the Catholic Church to create confrontation with the political establishment to give the impression they're being persecuted," she said.

"There's nothing like persecution to get the believers onside."

The Archbishop's comments follow those of Cardinal Keith O'Brien, the leader of the Catholic Church in Scotland who compared gay marriage to legalising slavery on Monday.

Huffington Post blogger Greg Jenner said the cardinal's argument was "self-destructive."

A Catholic lesbian also argued against Cardinal O'Brien, saying in the Daily Record that her uncle, a Catholic priest, had reassured her about her sexuality.

In response to the Archbishop's alleged comments, Andy Wasley of Stonewall said: "No one is proposing to force any religion to conduct same sex marriages, our very strong advice to anyone who doesn't support gay marriage is you shouldn't marry someone of the same sex."

The former Archbishop of Canterbury Lord Carey has also spoken out against gay marriage, writing in the Daily Mail: "Marriage precedes both the state and the church, and neither of these institutions have the right to redefine it in such a fundamental way."

However equalities minister Lynne Featherstone has said the Church does not "own" marriage, arguing in February: "This is not a battle between gay rights and religious beliefs. This is about the underlying principles of family, society and personal freedoms."

A public consultation "to consider how to make civil marriage available" is scheduled to begin this month.