When Kate Winslet landed a Best Actress Oscar for The Reader with her sixth nomination last year she knew exactly whom to thank. Shaking with emotion she told the watching world: “I’m so lucky to have a wonderful husband and two beautiful children who let me do what I love and who love me just the way that I am.”

She had said much the same thing on lifting a Golden Globe a few weeks earlier for her performance in Revolutionary Road, directed by that husband, Sam Mendes.

She told Mendes: “I can honestly say with my hand on my heart that I loved every second of working with you and it has made me love you more.”

To outsiders they appeared, until yesterday, the high-achieving couple who had it all: wealth, the deep respect of their peers, a solid family life out of the spotlight and even the confidence to work together on Revolutionary Road, which anatomised a marital breakdown.

In private their own marriage had been under strain for some time and last night the couple’s lawyers announced that they had separated.

Keith Schilling, of the legal firm Schillings, said in a statement: “Kate and Sam are saddened to announce that they separated earlier this year. The split is entirely amicable and is by mutual agreement. Both parties are fully committed to the future joint parenting of their children.”

No third party is known to be involved. Rather, in an uncomfortable echo of the film that they made together, one of the couple is believed to have yearned for a more exciting lifestyle than the one they had carved out for themselves in Gloucestershire and New York.

Revolutionary Road explores similar territory. Based on Richard Yates’ novel, which tells the grim story of April and Frank Wheeler, a married couple in 1950s America who become frustrated with the constrictions of their seemingly comfortable family life, with tragic consequences.

Winslet is understood to have leant heavily on her co-star in the film, Leonardo Di Caprio, for advice in recent months, although there is no suggestion of any romance between the pair. Both actors became huge global stars together as the leads in Titanic in 1997 and have remained close friends ever since.

Winslet grew up in a terrace house in Reading, Berkshire, and was bullied at school because of her weight. Encouraged by her parents, who are actors, she auditioned for television roles and at the age of 11 appeared alongside the Honey Monster, the fluffy yellow creature used to advertise Sugar Puffs.

Her first break into feature films came in 1992 when Peter Jackson picked her to appear in Heavenly Creatures. Her portrayal of a teenager who conspires to murder her friend’s mother began a rapid rise to stardom.

A year after Heavenly Creatures was released, she appeared as Marianne Dashwood in Sense and Sensibility, a role for which she received her first Oscar nomination. Her second nomination came two years later, in 1998, for the lead in Titanic, a film that was the most successful until its box office receipts were overtaken by Avatar.

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