In a major intervention, former British Prime Minister Tony Blair has made clear he “strongly supports” gay marriage and disagrees with Pope Benedict XVI on the issue.

Irish-born Cardinal Keith O'Brien, 73, the leader of the Catholic Church in Scotland, is leading the battle to defeat a gay marriage bill in the British parliament.

He has called gay marriage "madness" and a "grotesque subversion of a universally accepted human right."

O'Brien, an Antrim native, said countries that legalize gay marriage are “shaming themselves” and "going against the “natural law.”

“Imagine for a moment that the Government had decided to legalize slavery but assured us that "no one will be forced to keep a slave."

“Would such worthless assurances calm our fury? Would they justify dismantling a fundamental human right? Or would they simply amount to weasel words masking a great wrong?”

He stated “I think it’s a very, very good example of what might happen on our own country in the present time."

He claimed that “further aberrations would take place and society would be degenerating even further than it already has into immorality.”

Blair, the most high-profile recruit to the Catholic Church in recent times, converted in 2007. He has now made clear he supports a new bill by Prime Minister David Cameron allowing gay and lesbian couples to marry.

On Friday, Pope Benedict called on all Catholics to block the "powerful political and cultural currents seeking to alter the legal definition of marriage."

However, the Independent on Sunday has reported that Blair says he "strongly supports the Prime Minister's proposal."

Blair's move will deeply anger the Vatican as he has remained silent on the issue until now.

When he converted, the Vatican stated his move, prompted by his wife’s Catholic faith, "can only arouse joy and respect."

The bill is said to have a good chance of passing.

A traditional Tory, Eric Pickles, the Secretary of State for Communities and Local Government, stated on Thursday: "I have to say I've changed my mind on this in recent years and I'm rather in favor of gay marriage."

Cardinal Keith O'Brien