X Files star David Duchovny is to sue the Daily Mail newspaper for $1 million (£617,000) in damages after it claimed he had an affair with his tennis instructor.

The Mail on Sunday claimed at the weekend that the 48-year-old actor and Edit Pakay had been involved in a "physical" relationship for at least six months.

Duchovny's lawyer Larry Stein rubbished the newspaper claims earlier - quoting Ms Pakay - and has since filed a lawsuit for defamation and invasion of privacy.

The Mail's story was removed from its website after complaints from Duchovny's representatives, but its reporting by a number of other websites caused "additional and further damage to David Duchovny," according to the suit filed in the Los Angeles superior court.

The suit also stresses that an affair between Duchovny and Ms Pakay "never happened" and claims they are merely friends, adding that Ms Pakay was a tennis partner of the actor's rather than his instructor.

Duchovny, who made his name as Fox Mulder in the X Files, voluntarily entered a facility for the treatment of sex addiction in late August and checked out in October, having "successfully completed his treatment".

However, it was announced last week that the actor and his wife Tea Leoni have been "separated for several months".

The couple have two children, a nine-year-old daughter Madelaine West, and a six-year-old son, Kyd.