A French court has backed footballer Karim Benzema as he fights to have a blackmail case over a sex tape dropped.

The Real Madrid forward was placed under formal investigation in 2015 over an alleged bid to blackmail fellow footballer Mathieu Valbuena.

When he and another suspect argued the police had acted dishonestly, their case was rejected by the Appeal Court.

Now another court has backed Karim Benzema's complaint. His lawyer says he expects the case to be thrown out.

"It's definitely a big victory," said lawyer Patrice Spinosi, who told France Info it was a turning point in the investigation.

He said Tuesday's decision by France's highest judicial court - the Court of Cassation - meant that the case would return to the Court of Appeal's examining chamber, which would have no choice but to drop the inquiry.

What did the police do wrong?

At the heart of the case was the role of an undercover police officer.

In 2015, a blackmailer got in touch with Mathieu Valbuena, threatening to make the sex tape public. The footballer was asked to appoint a go-between and at that point a police officer was given the task of negotiating on Mr Valbuena's behalf under the false name Lukas.

Three suspects were linked to the blackmail attempt before one of them approached a childhood friend of Karim Benzema for help. Karim Benzema is then alleged to have discussed the sex tape with Mathieu Valbuena during a France team training session in October 2015.

According to Mr Spinosi, the undercover officer had played a "very active role", pushing the two sides towards a deal. Without his involvement no offence would have taken place, he believes.

Prosecutors argue that Karim Benzema urged his colleague to pay the blackmailers although he denies the allegation.

Is the case over?

Even if it is eventually thrown out, that could take months.

The Court of Cassation has only ruled on the conduct of police during the investigation rather than on the allegations themselves, commentators say. That will be the task of the Court of Appeal.

Another of Karim Benzema's lawyers, Sylvain Cormier, said "we aren't forgetting the main battle is to prove his innocence".

Since the affair emerged in November 2015, neither footballer has played for their national team.

Karim Benzema joined Real Madrid in 2009 and is seen as one of the club's leading players, helping them to Champions League victory last month in Cardiff.

Mathieu Valbuena's Lyon team finished fourth in France's Ligue 1 last season and reached the semi-finals of the Europa League.