Gay rights charity Stonewall has published a draft marriage bill which it says contains the provisions necessary to extend equal marriage rights to same-sex couples.

The bill would retain civil partnerships for gay couples and give civil partners the option of converting their union to a marriage.

The two-page bill lists five legislative steps necessary to implement equal marriage and comes ahead of the government’s promise to open a consultation in March of this year on how to introduce equal civil marriage.

Ben Summerskill, Chief Executive of Stonewall said: “We trust that the government will go ahead with its promised consultation by the end of March.

“We also trust that the government will publish a White Paper, consulting on the detail of how to implement its proposal, and not a Green Paper which merely consults on the wider principle. This simple two-page Bill of five clauses need not occupy a significant amount of parliamentary time. It could easily be included in the Queen’s Speech in May.”

The draft bill includes exemptions for religious institutions which have been a constant concern in the equal marriage debate.

The regulations allowing for civil partnerships to take place in religious institutions which elect to hold them was subject to a failed last-minute challenge in December questioning the protection they afforded to places of worship.

In the case of civil partnerships, concerns that the specific exemptions afforded to religious institutions would be overridden by broader equality legislation were rejected by legislators and many commentators.

The Church of England’s legal office confirmed the exemption was sufficient as the institutions of marriage and civil partnerships were distinct, so it was “clearly” not discriminatory to offer one but not the other.

Stonewall had come under pressure from co-founders and other gay campaigners to adopt a position in favour of equal marriage, doing so in 2010.

An open letter sent to the charity in 2010 cited the findings of a PinkNews.co.uk poll which found 98% of readers favoured equal marriage rights.

In a statement posted on its website, the charity said it was “pleased to be widening its campaigning objectives to include extending the legal form of marriage to gay people.

“We seek to secure marriage for gay people as a civil vehicle on the same basis as heterosexual marriage, available in a registry office but without a mandate on religious organisations to celebrate it.

“We seek to retain civil partnerships for lesbian and gay people recognising their special and unique status.”

The Stonewall draft marriage bill released today does not make provision for straight civil partnerships, which have been advocated by some campaigners, but the charity said this is because it only represents the interests of gay and bisexual people.

On the question of whether civil partnerships should be opened up to straight couples, Stonewall’s draft marriage consultation response says: “This is a matter for heterosexual people and Stonewall would recommend consulting with them and stakeholder organisations representing them.”