Clint Eastwood lent his support to the fight for gay marriage, adding his name to a legal brief that calls on the US supreme court to legalise same-sex unions. The Oscar-winning film-maker and actor made an eccentric appearance at last year's Republican national convention, berating an empty chair to the various amusement and bemusement of the gathered dignitaries. He now joins a breakaway group of moderate Republicans that threatens to expose a split within the party.

Eastwood is a resident of the state of California, where a constitutional amendment, Proposition 8, currently limits marriage to a man and a woman. Next month the supreme court is due to hear arguments about the legality of the ban. The arguments could eventually lead to the proposition being overturned and gay marriage being legalised.

The 82-year-old director of Unforgiven and Million Dollar Baby was one of more than 80 Republicans who signed the "friend of the court" brief, which was then released by the American Foundation for Equal Rights. Adam Umhoefer, the foundation's executive director, described the brief as "a microcosm of what we see happening all across the country". He added: "Americans are united in the concept of freedom, dignity and strong families."

A recent poll found that 61% of California voters are now in favour of same-sex marriage.