The split shocked Tinseltown where they were long regarded as having one of the strongest relationships in the entertainment industry.

Sarandon, 63, and Robbins, 51, worked together when he directed her in the 1995 film Dead Man Walking, for which she won an Oscar. Robbins later won an Academy Award for best supporting actor for his role in Mystic River, which was released in 2003.

They are also known as a “political power” couple, with both speaking out against the Iraq war in 2003 and publicly supporting the Democrats in last year’s election.

Sarandon’s publicist Teal Cannaday said: “Susan and her partner of 23 years, Tim Robbins, have announced that they separated over the summer.”

Sarandon, whose latest film The Lovely Bones is due out for release in the UK next month, met Robbins while making the 1988 film Bull Durham, in which Robbins played a baseball player and Sarandon the fan who simultaneously seduced him and prepared him for the big leagues.

They never married but went on to have two sons together. Sarandon also has a daughter from a previous relationship with the Italian filmmaker Franco Amurri.

In 2005 Sarandon flew to Scotland to introduce Bob Geldof at the Live 8 concert in Edinburgh. She described the choice of Sarah Palin as the Republicans’ vice-presidential candidate “divisive and cynical”.

In 2004, Sarandon said: “Tim is a guy guy, and he is also 12 years younger than me. I won’t marry because I am too afraid of taking him for granted or him taking me for granted – maybe it will be a good excuse for a party when I am 80.”

She later told The Herald: “I like it when somebody does something thoughtful, like framing a great picture – when somebody takes the time to listen to you, and maybe constructs something.

“Anything that’s not just going out and shopping – not that I’d turn down jewellery – but I think romance takes a little bit of creativity.”