He jokingly apologised to his LGBT guests that he had not brought US President Barack Obama back home with him, which prompted one lady to shout: "What about Michelle?"

The Prime Minister then commented on the passing of Proposition 8 in California last November.

Prop 8 overturned a state Supreme Court ruling in May 2008 that legalised gay marriage in California.

The court heard legal arguments about the validity of the ballot measure earlier today.

"What I saw in America tells me what we have got to do," Mr Brown said.

"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.

"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."

The reception was held to mark LGBT History Month, which is celebrated every February.

It was due to be held last week but was postponed out of respect for the Leader of the Opposition, David Cameron, whose six-year-old son Ivan died last Wednesday.

Gay celebrities including Radio 1 DJ Scott Mills, Erasure singer Andy Bell, artist Maggie Hambling and East Enders star turned Labour MEP Michael Cashman attended.

Comedian Amy Lamé was there with her girlfriend Jenny, celebrating their Hen Night.

The Prime Minister congratulated them on their civil partnership, which will be held tomorrow.

LGBT Labour praised Gordon Brown's comments about Prop 8.

"I am so proud that our Labour Prime Minister is the first world leader to come out passionately against Prop 8," said Richard Angell, the group's national secretary.

"Tonight's event embodied the wholesale change that has taken place over the last 12 years."

END