Six out of the seven parties in Sweden's parliament have now said they are in favour of gay marriage, which has raised the likelihood of new legislation dramatically.

But the fourth member of the coalition, the Christian Democrats, refused to back the bill which will now need the support of at least one of the centre-left opposition parties.

The three opposition parties said they were ready to work together with the centre-right parties, with the aim of having the new legislation in place as soon as possible.

"We are very pleased to finally be able make these changes," Moderate party MP Lars Lindblad told a news conference.

"There is a firm basis for a broad agreement between six parties," he added, referring to a possible deal including all parties in the Swedish Riksdag except the Christian Democrats.

The change in the law, which currently allows gay couples to register unions but not formal marriage, would come into force by May 1 this year under the timetable set out in the bill.

The proposed new legislation would eliminate legal distinctions between heterosexual and homosexual spouses, but it would not force dissenting clergy to wed gay couples.

The Swedish Lutheran church, which split from the state in 2000, has said it was open to celebrating and registering same-sex unions, though it wanted to reserve the term matrimony for heterosexual marriages.

The church already performs blessing ceremonies for gay couples who have registered unions with civil authorities after a church council decision in 2005 which outraged conservatives.