By Lee Carter BBC, Toronto

Michael Stark and Michael Leshner were able to marry immediately

The ruling upholds a lower court decision and challenges the Canadian Government to change its laws on same-sex marriage.

The appeals court ordered that gay couples seeking a legal union should be issued a marriage licence immediately.

The case had been fought by a gay couple, Michael Stark and Michael Leshner, who wasted no time in getting married hours after the ruling.

I'm very clearly opposed to redefining marriage Liberal MP Pat O'Brien

Another couple, Jeff Parker and David Wood, saw the ruling on the internet and immediately went to Toronto City Hall to get their papers ready for an August wedding.

The 61-page ruling says the heterosexual definition of marriage violates gay couples' rights under the country's constitutional document, the Charter of Rights and Freedoms.

Ontario is the third Canadian province to strike down the federal heterosexual definition of marriage in court decisions.

Public opinion

The Canadian Government had asked for more time to study the issue. Now it may not have any left.

All three Liberal party leadership candidates to replace outgoing Prime Minister Jean Chretien said they would not contest the ruling.

"One thing is very clear that the government cannot discriminate when it's an issue of rights and that's what the courts are in the process of deciding," said one of the candidates, Paul Martin, who is widely seen as the favourite to become the next prime minister.

However some government and opposition members of parliament said they were dismayed by the decision and planned to challenge it at federal level.

"I'm very clearly publicly already opposed to redefining marriage. There's no way I could possibly support that in good conscience and will not support that," said Liberal federal MP Pat O'Brien.

Polls indicate a slight majority of Canadians favour legalisation of same-sex marriages. Some Anglican churches in Canada have already offered blessings to gay and lesbian couples.