Chris Brown's lawyer has called for a probe into the police's handling of the assault case involving the R&B star and his girlfriend, Rihanna.

Mark Geragos filed a motion this week claiming a photograph of Rihanna with facial injuries had been sold by police to celebrity website TMZ.

The photographed was apparently taken by police as part of an investigation into an altercation between the couple in February.

Chris has been charged with assault over the incident in Los Angeles and is due to appear in court next month.

In the motion, Geragos wrote: "The purpose of the leak was necessarily for profit and to vilify Mr Brown and poison the potential jury pool.”

If discovered, police misconduct could trigger grounds for the case against Chris to be dismissed, the singer's lawyer said.

The couple have kept a low profile since the incident in February, which took place hours before the Grammy awards.

Rihanna made her first major public appearance earlier this week at the Costume Institute Gala in New York.

Chris, who celebrated his 20th birthday on Tuesday, could face up to five years in jail if found guilty.

Chris Brown and Rihanna