French footballer Karim Benzema has been placed under formal investigation in connection with a sex tape blackmail plot involving another player.

The Real Madrid striker appeared before a judge in Versailles after being held in police custody overnight.

Police are investigating what part he played in an alleged attempt to extort money from fellow French international Mathieu Valbuena.

A source told AFP news agency that he had admitted involvement.

The 27-year-old star told investigators he had approached Mr Valbuena about the tape on behalf of "a childhood friend," the report said.

According to French media, Mr Benzema mentioned the tape to the midfielder during a national team training session at Clairefontaine in early October, ahead of matches against Armenia and Denmark.

It was unclear whether he was offering friendly advice or trying to press his team-mate to pay the blackmailers.

The prosecutor's office has confirmed that Mr Benzema is now under formal investigation for complicity in attempted blackmail and participation in a criminal conspiracy.

When a suspect is placed under formal investigation, he or she is then examined by a judge, who determines whether there is sufficient evidence for the suspect to be charged.