The British stars, who met on a flight to Cannes in 2001 and split their time between homes in London and Hollywood, are parents to a 5-year-old daughter, Gracie.
Talk of trouble for the twosome first started last December when Friel, who recently appeared in Woody Allen's "You Will Meet a Tall Dark Stranger," was photographed kissing actor Joseph Cross, 24, her costar in the West End production of "Breakfast at Tiffany's."
Even after Cross moved into the duo's Hollywood home in April, the actress insisted they were just pals, saying, "Joseph is a good friend of mine. I have no problem with him staying at my house."
And while Thewlis, who stars as werewolf Remus Lupin in the "Harry Potter" films, has said, "Anna changed me for the better, from a rather wild, restless character into a gentle soul," Friel has admitted their relationship is somewhat unconventional.
"He's 13 years older than me," she has said. "We have our own separate properties and we're not married."
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