Miller, who has starred in the films "Factory Girl," "Casanova" and "Alfie," is best-known for her stormy relationship with actor Jude Law. While they were engaged, Law publicly admitted having an affair with his children's nanny.
Law and Miller got back together and broke up several times before calling it quits last year.
The Daily Mail reported Miller was seen "locked in a tight embrace" with P. Diddy, whose real name is Sean Combs, at a party Sunday after the "Concert for Diana."
Combs' longtime girlfriend, actress Kim Porter, reportedly broke up with him recently because she suspected he and Miller have been more than just friends since they met earlier this year, the Daily Mail said.
Combs previously left Porter for several years to date singer/actress Jennifer Lopez but the couple reconciled when his relationship with Lopez fizzled.
Sources close to Porter told the New York Post she has moved out of his New York apartment and has no plans to return.
A spokesman for Combs denied the story was true, the Post said.