A leading research centre has estimated that more than 18,000 gay and lesbian couples have got married in California since the state Supreme Court ruled in favour of same-sex marriages in May.

Today voters in the state will consider a ballot measure, Proposition 8, that would overturn the court’s ruling and remove the right to marry from gays and lesbians.

There are also votes to ban same-sex marriage in Florida and Arizona, but the focus of concern is California, the most populous state in the US.

The Williams Institute at UCLA School of Law estimates that by Election Day approximately18,000 same-sex couples will have married in California.

Same-sex couples have been legally permitted to marry in California since June 17th 2008.

Marriage licences in California do not include the sex of spouses.

The Williams Institute’s estimate is primarily derived by comparing total marriages in the state in 2007 to marriages in 2008 and assuming that all increases are attributable to marriages by same-sex couples.

The Williams Institute is a national think tank dedicated to research on issues of sexual orientation law and public policy.

An estimated $60m (£32m) has been donated on both sides of the Prop 8 debate – Hollywood stars such as Steven Spielberg and Brad Pitt have donated large sums to the campaign opposing it.

A recent Field Poll found 49% of likely voters oppose Proposition 8 and 44% support it, with 7% undecided.

Proposition 8 would amend the state Constitution to “eliminate right of same-sex couples to marry.”

In May the California Supreme Court overturned a ban on same-sex marriages in the state.

The Court voted 4 to 3 to strike down the ban, ruling that lesbian and gay couples are entitled to the same fundamental right to marry as heterosexual couples as protected by the California constitution.

The decision explicitly struck down Proposition 22, a voter-passed initiative in 2000 that sought to limit marriage to between a man and woman.

Presidential candidate Barack Obama is expected to win California and its 55 electoral college votes comfortably.