When her ex-boyfriend cheated on her, Sienna Miller threatened some rather menacing revenge on the other woman.

So, on the basis of what goes around comes around, the 25-year-old actress had better watch her back.

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Caught out: Sienna Miller and P-Diddy get up close and personal at the Concert for Diana afterparty

The pair had dinner at Cipriani's restaurant, where he is said to have serenaded her in front of guests

The British star has been blamed for the break-up of American rapper P Diddy's long-term relationship with the model Kim Porter, the mother of his three children.

According to reports in the U.S., 36-year-old Miss Porter has moved out of the couple's New York apartment following rumours concerning the nature of Diddy and Miss Miller's friendship.

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The pair first met at a film festival in Utah were they spent the night huddled together in conversation

News of the split comes just five days after the rapper and Miss Miller spent much of the evening flirting in the VIP area of London's Paper nightclub after the Princess Diana memorial concert.

As Miss Miller walked into the club, she approached Diddy - real name Sean Combs - and flung her arms around him in a dramatic embrace before appearing to kiss him full on the lips.

They recently also enjoyed a romantic dinner together at the London restaurant Cipriani.

Six months ago, shortly after meeting Miss Miller at a film festival, the singer was caught sneaking into the actress's Manhattan hotel at 9am following a night of clubbing.

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Diddy with his long-suffering girlfriend Kim Porter in happier times

When photographers captured them together, just a few weeks after Miss Porter gave birth to twin daughters, Diddy's security team muscled in and insisted that the offending pictures be deleted.

Despite persistent rumours to the contrary, both Miss Miller, and 37-year-old Diddy have denied any romance.

A source told the New York Post: 'Kim is in Louisiana filming a movie right now, and when it's done she's moving to L.A. She is focusing on her career as a model and actress and raising her children. They are still close, but they just can't be together right now.'

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Sienna Miller looked subdued as she arrived at her hotel in Rome, Italy yesterday

Miss Miller split from Jude Law last year after he had an affair with Daisy Wright, his children's nanny.

Refusing to comment on the infidelity at the time, Miss Miller recently broke her silence, however, saying: 'Daisy better live in fear. I'm quite looking forward to the days when our paths will cross, which I know they will. I just hope that woman doesn't run into me in a dark alley.'

Last night, a spokesman for Diddy denied that he and Miss Porter had split, saying: 'There is no truth to this.'