TWENTY faith leaders have signed a letter urging people to declare their support for same sex marriage to two federal parliamentary inquiries on the issue.

The move follows six Victorian Catholic bishops writing to their parishioners to tell them allowing gay couples to marry would be a ''grave mistake'' and would undermine the institution of marriage.

Signatories of the pro-gay marriage letter, organised by the gay marriage campaigners Australian Marriage Equality, include the Sydney Uniting Church minister Bill Crews, the Baptist minister Mike Hercock, the Metropolitan Community Church pastor Karl Hand and the Outreach Ministries church fellowship senior pastor Shane Anderson.
 
The letter reads: ''As clergy from various different faiths and denominations in Australia, we believe marriage is a fundamental institution in our society. It fosters greater commitment between partners, provides children with a sense of security and stability, and strengthens ties with families and communities.''

Three private members bills to legalise gay marriage are before federal Parliament. 

Submissions to the inquiry being held by the Senate legal and constitutional affairs committee close today, while submissions to the inquiry being held by the House of Representatives social policy and legal affairs committee close on April 20.

As things stand, each bill appears headed for defeat by its Labor opponents - including the Prime Minister, Julia Gillard, - and the Coalition.

While Labor MPs have a conscience vote on the issue, Coalition MPs do not.