Clint Eastwood has joined over 100 top Republicans who have signed up to argue that the Supreme Court should overturn Proposition 8, which bans equal marriage in the state of California.

The veteran actor and director signed the amicus curiae brief, an official document to be filed with the Supreme Court, which was also signed by members of George W Bush’s cabinet, former governors, and some who had changed their stance on marriage since Prop 8 was introduced in 2008.

On Monday,around 75 leading Republican figures had signed the brief urging the Supreme Court to strike down Proposition 8, in a move which could have a significant effect on the Republican Party and equal marriage in the US. The list has now grown to over 100.

A friend of the court brief has been filed to the US Supreme Court by the Republicans.

The Supreme Court is due on 26 March to take up the case of whether to overturn Proposition 8, which in 2008 added a clause to the Californian constitution stating that marriage could only be recognised by the state if it were between a man and a woman, causing widespread controversy.

The list does include a crowd of high-profile Republicans, including some who had previously opposed equal marriage, such as Meg Whitman, who ran for the Governor of California pledging to uphold Proposition 8, and former Congresswoman Deborah Pryce.

Ms Pryce said: “Like a lot of the country, my views have evolved on this from the first day I set foot in Congress. I think [equal marriage] is just the right thing, and I think it’s on solid legal footing, too.”

Former Utah governor and potential 2016 presidential candidate Jon Huntsman appears on the list.

Last week, Mr Huntsman wrote a piece for American Conservative magazine entitled “Marriage equality is a conservative cause”.

Back in September, Clint Eastwood was a guest speaker at the Republican National Convention, despite his pro-marriage equality stance. Details, images and videos of his speech went viral because he spoke to an empty chair as if it was President Obama, who tweeted following the speech saying he was still a fan.

In a 2011 interview with GQ, the oscar-winning actor and director passionately explained why he thought people were “making a big deal” out of equal marriage, and why he was in support of it. 

The briefing follows the model of one filed earlier this week by leading lawyer and former solicitor general Ted Olson, who argues that Proposition 8 damages family values by denying the dignity of the thousands of American families with same-sex parents.