The Prime Minister has attacked as “unacceptable” a measure reserving marriage for one man and one woman passed by public vote in California.
Gordon Brown raised the issue at a Downing Street reception to celebrate LGBT History Month.
Proposition 8 was passed by a public vote in November, overturning an earlier court ruling allowing gay marriage in the state of California.
It amended California’s state constitution making it clear that only a marriage between a man and a woman is valid or recognised in California.
But Mr Brown, who had just returned from his visit to meet the new US President, Barack Obama, said: “What I saw in America tells me what we have got to do.
“This Proposition 8 in California, this attempt to undo good that has been done, this attempt to create divorces for 18,000 people who were perfectly legally brought together in partnerships.
“This is unacceptable and this shows why we have always got to be vigilant, always got to fight homophobic behaviour and any form of discrimination.”
Although the UK allows civil partnerships, the Government has stopped short of calling them ‘marriage’.
Colin Hart, Director of The Christian Institute, said: “Mr Brown’s comments call into question the Government’s commitment to marriage as between one man and one woman.”
LGBT History Month, for which the reception was held, has taken place every February since 2005. It was conceived by ‘gay rights’ campaigners with the aim of educating pupils to recognise homosexuality as normal.
This year’s celebrations have seen schoolchildren in Kent asked by their local police to write essays on their feelings about homosexuality, and pupils in London performing a production of Romeo and Julian.
The rainbow flag – a symbol for the gay rights movement – was flown outside police buildings until officials removed it, reportedly saying that the police should not “stray into political territory”.
In previous years the campaign tried to teach children that Florence Nightingale was a lesbian and that Isaac Newton was gay.
The organisers commend one school which in 2005 reportedly forced several Christian teenagers to attend an LGBT History Month event despite their objections.
Mr Brown congratulated the LGBT History Month organisers at the reception: “I want to say to you all, you have broken new ground, you have shown what can be done, you have shown how you can change opinion in our country, you have shown how the legislative process, by your pressure, can respond.”
Richard Angell, the national secretary of LGBT Labour, said: “I am so proud that our Labour Prime Minister is the first world leader to come out passionately against Prop 8.
“Tonight’s event embodied the wholesale change that has taken place over the last 12 years.”