Responding to news that ministers will hold a consultation on giving gay couples the right to civil marriage, chief executive Ben Summerskill it would have to start now for changes to be implemented by in this parliament. Stonewall only added gay marriage to its campaigning objectives last October, eight months after the Liberal Democrat leader and deputy prime minister Nick Clegg declared his support for a change in the law.

Liberal Democrat equality minister Lynne Featherstone and other leading Liberal Democrat and liberal Conservative ministers are understood to be keen for gay couples to be able to marry.

In his strongest statement yet on the matter, Mr Summerskill said: “Stonewall fully supports extension of the legal form of marriage to lesbian and gay couples.

“We’re told that this will undermine the nature of marriage. However there’s no evidence that, if marriage is available to gay people, a single heterosexual will end up choosing to marry someone of the same sex, either by design or by accident.

“If there’s a genuine commitment to making progress in this area, it is painfully slow. Equalities minister Lynne Featherstone has explicitly said she would consult on proposals the government intends to implement in the lifetime of this parliament. If that is to happen by 2015, then consultation should begin now.”

Stonewall was instrumental in passing 2005′s civil partnership legislation but was slow to publicly support gay marriage.

The charity was heavily criticised in September, even by some of its own co-founders, Michael Cashman MEP and Sir Ian McKellen, for appearing not to back the campaign while figures such as deputy prime minister Nick Clegg and Labour leader Ed Miliband had declared their support in articles for PinkNews.co.uk months before.

Last October, Mr Summerskill said the charity would not be “jumped into” declaring a position on the issue. In 2009, Mr Summerskill told PinkNews.co.uk that “lots of” gays and lesbians do not want the right to marry.

Gay marriage campaigners planned to protest at Stonewall’s annual awards in November but called off the demonstration after the charity announced that after surveying its supporters, it would add the issue to its campaigning objectives.

Stonewall has said it will not lobby for straight people to have civil partnerships, as campaigning for heterosexual rights is not part of its remit. It claimed that it could cost up to £5 billion in lost tax revenue.

The charity cited the need to ‘consult’ its supporters before officially campaigning for gay marriage. The government says it needs to complete a consultation on proposals to bring in gay marriage equality before tabling any proposed legislation.