The Obama administration is to file a brief asking the Supreme Court to strike down California’s ban on equal marriage, and to take the view that other, similar bans, are unconstitutional.

According to a source familiar with the Obama administration’s briefing on the Proposition 8 case, the controversial legislation which bans equal marriage in the state, lawyers plan to file a brief before the deadline at midnight on Thursday.

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 brief is also to contain suggestions that cases of discrimination based on sexual orientation should be treated with extra scrutiny by the court.

The argument by the administration is that denying gay and lesbian people the right to marry is unconstitutional, as it violates the Constitution’s Equal Protection Clause.

Reports suggested that the source within the administration spoke anonymously in order to discuss the document before it had been officially filed.

The brief is to be filed on Thursday, which is also the deadline for such a filing to be made.

Earlier today, Clint Eastwood joined over 100 top Republicans who signed up to argue that the Supreme Court should overturn Proposition 8.

A new poll released this week revealed that 61% of Californians support the right of same-sex couple to marry, a record percentage of support.

In a similar filing made last Friday by Obama’s office, the administration urged the Supreme Court to strike down the Defense of Marriage Act, also saying courts considering laws targeting gay people should do so with extra scrutiny, and calling it unconstitutional.