Tony Blair has defied the Pope by making clear he "strongly supports" plans for gay marriage, The Independent on Sunday has learnt. The former prime minister, a Catholic since 2007, backs proposals by David Cameron for a new law allowing homosexuals to marry.

A consultation on gay marriage to be launched later this month has met fierce opposition from senior figures in the Roman Catholic Church and the Church of England.

On Friday, Pope Benedict XVI urged Catholics to block the "powerful political and cultural currents seeking to alter the legal definition of marriage". It followed an attack last week by Cardinal Keith O'Brien, the leader of the Catholic Church in Scotland, who said allowing same-sex marriage was "madness" and a "grotesque subversion of a universally accepted human right".

The Archbishop of Canterbury, Rowan Williams, and John Sentamu, the Archbishop of York, are also opposed. Last night Dr Williams visited the Pope for private talks at the Vatican, but it was not disclosed whether they discussed the Government's proposals.

Today the campaign against gay marriage is stepped up with a letter from two archbishops to be read to Catholic congregations across England and Wales at mass. The letter will claim that the new law would undermine the institutions of marriage and the family.

But despite the backlash, the Prime Minister and many members of his Cabinet are fully behind the plans. In a significant boost to Mr Cameron, Mr Blair – one of the best known Catholics in Britain – has told friends he "strongly supports the Prime Minister's proposal".

Mr Blair's position is likely to anger the Vatican, which is alarmed at moves in both the UK and the US for same-sex unions. Senior church figures are also opposed to church blessings of same-sex civil partnerships. Mr Blair was responsible for pushing through laws on civil partnerships and lowering the age of consent for homosexuals to 16, but he has remained silent on the issue of gay marriage until now.

When Mr Blair joined the Catholic Church, six months after leaving Downing Street, the Vatican's chief spokesman, Federico Lombardi, said his conversion "can only arouse joy and respect". He joined because his wife, Cherie, is a Catholic and their four children were brought up in the church. But Ann Widdecombe, the former Tory minister and also a Catholic convert, complained that his record as prime minister clashed with the church's teachings.

Lynne Featherstone, the equalities minister who will launch the consultation on the new law, has pointed out that marriage is "owned by the people" and not the church. Throwing his weight behind the plans at his party conference last year, Mr Cameron said: "Society is stronger when we make vows to each other and support each other. So I don't support gay marriage despite being a Conservative; I support gay marriage because I'm a Conservative."

Even Tory traditionalists have spoken out in favour of gay marriage. Eric Pickles, the Secretary of State for Communities and Local Government, said on Thursday: "I have to say I've changed my mind on this in recent years and I'm rather in favour of gay marriage."