Narnia star Tilda, her stay-at-home partner ... and her intriguing other life with a 'delightful painter'

But now she has confessed that her private life is just as extraordinary: she is enjoying a relationship with a "delightful painter" while still living with her long-term partner and their two children.

The 47-year-old Chronicles Of Narnia star has admitted to the unusual double life following a spate of internet rumours.

Swinton lives with John Byrne, a painter, director and writer, and their ten-year-old twins in the Scottish Highlands, an hour north of Inverness, yet spends much of her time away from home with a mystery partner, identified only as another painter.

Despite her new relationship she insists that she and Byrne, who is 20 years her senior, remain "the best of pals".

Meanwhile, Byrne has told The Mail on Sunday the "extraordinary" arrangement has his full blessing.

Cambridge-educated Swinton, who is descended from an aristocratic family, told the Los Angeles Times: "We are the best of pals and adore being parents and are devoted to that project.

"We ostensibly live in the same house but I travel the world with another delightful painter. The arrangement is just so sane."

She has been photographed several times in recent months hand-in-hand with a mystery man, thought to be the unnamed painter.

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Byrne, who began his relationship with Swinton in 1990, last night confirmed: "We're amicably living together in the same house, under the same roof. It's extraordinary. We love each other too, in an extraordinary way.

"We have always been the best of chums and will remain the best of chums for the rest of our lives. We love our children and our children love us. The rest is no one else's business.

"If one is getting on with one's life, other people's affairs and how they want to live is entirely their affair. Tilda explained it very well indeed. There's nothing I can add to that.

"I endorse [her statement] wholeheartedly. That's as much as I think she wants you to know, and as much as I want you to know."

He declined to comment on the identity of the "delightful painter".

When asked whether he also had another relationship, he said: "I hate to be discourteous but I think that's our business."

Swinton attended West Heath Girls' School in Sevenoaks, Kent, along with a young Princess Diana, and later went to Fettes College in Edinburgh, Tony Blair's old school.

She met Byrne on the TV series Your Cheatin' Heart, which gave the actress her first major role.

Her big film break came in 1992, when she took the title role in the art-house hit Orlando, and in 2000 she achieved Hollywood stardom when she appeared alongside Leonardo DiCaprio in The Beach.

Her other film roles have included Constantine, Adaptation and Michael Clayton, which earned her a Golden Globe nomination this year for Best Supporting Actress.

When asked about Swinton's relationship with the "delightful painter", a spokesman for her agent said last night: "There is nothing I can say. I cannot comment on that at all."

He later added: "To clarify, Tilda is not married to John Byrne."