The BBC's Rob Corbett: "The decision is expected to bring about a rewriting of laws across Canada"

The ruling makes Canada's laws among the most favourable in the world for homosexuals.

The decision stems from a refusal by a court in Ontario to recognise the entitlement of a lesbian - known only as M - to receive maintenance payments after the end of a relationship with her partner, H.

M asked the courts to strike down the provision in Ontario's marital law that limited the right to claim maintenance to married or common-law heterosexual couples.

The Supreme Court ruled that the relationship between the two women was clearly a spousal one.

The court stressed that it was not changing "traditional conceptions of marriage," and noted that the Ontario law had already expanded spousal support beyond married persons, to heterosexuals living in common-law relationships.

But it said its decision in this case "may well affect numerous other statutes that rely upon a similar definition of the term 'spouse'".

Ontario must amend its laws to include same-sex couples within six months. The ruling might also mean that hundreds of laws will also have to be rewritten in other provinces, and in federal legislation.

'Promise of equality'

Legal analysts believe the ruling brings same sex marriages a step closer.

Martha McCarthy, M's lawyer, praised Canada's 1982 Charter of Rights and Freedoms, on which the decision was based.

"It ... is the first time that gays and lesbians can say that the Charter's promise of equality for them and for their relationships has been fulfilled," she said.

Her client had already settled out of court with H, Ms McCarthy said, but Ontario had continued the case on principle.

In a written statement, M said: "Frankly the process has often made a root canal without an anaesthetic seem like a good time."

'Centuries of tradition overturned'

The judgement has been condemned by organisations supporting traditional family values.

"The Supreme Court has overturned centuries of social tradition," said Darrel Reid, president of Focus on the Family Canada, which argued against expanding gay rights.

But John Fisher, head of the homosexual rights group Egale, said: "It's about recognition. It's about respect. It's about a more equal Canada."

However each of Canada's provinces and the federal government have the right under the Charter of Rights to invoke a rarely used clause to say they will ignore the landmark decision.

The conservative Canada Family Action Coalition has launched a campaign to get the provinces to do just that.