British Prime Minister Gordon Brown supports the defeat of a California gay marriage ban argued yesterday in the state's highest court, reports the BBC.

At a Downing Street reception to celebrate LGBT History Month attended by prominent gay leaders, Brown called California's Proposition 8 “unacceptable.”

Brown's own party, the Labour Party, legalized civil unions for gay and lesbian couples in 2005. The UK has also allowed open gay service in the military since 2000.

Proposition 8 reversed a California Supreme Court ruling allowing gay marriage in the state when it defined marriage as a heterosexual union in the state constitution. Voters approved the measure by a narrow 52%. Yesterday, the state Supreme Court heard arguments in a lawsuit that seeks to void the measure. A decision is expected in 90 days.

Discussing Proposition 8, Brown said “this attempt to undo good that has been done is unacceptable.”

“This shows why we have always got to be vigilant, always got to fight homophobic behavior and any form of discrimination,” the prime minister added.

He also praised UK gay rights leaders for “changing opinion” about civil unions for gays and lesbians.

“You have shown how the legislative process, by your pressure, can respond,” Brown said.