Pop superstar Rihanna has been ordered to testify against ex-boyfriend Chris Brown during his forthcoming assault trial.

The Disturbia singer has reportedly been served with a subpoena to appear in a Los Angeles courtroom on June 22.

According to People magazine, Rihanna’s attorney Donald Etra confirmed the 21-year-old Barbadian beauty intends to co-operate fully .

“Rihanna was served at my office by L.A. DA investigators,” Etra told the magazine.

“If the preliminary hearing indeed goes forward, she is now legally required to be there, she will be there, and will answer all questions truthfully.”

In April, R&B singer Brown, 20, pleaded not guilty to one count of assault and one count of making criminal threats.

The accusations relate to an alleged domestic violence incident involving Rihanna in February.

Brown, meanwhile, has reportedly failed in an attempt to delay a preliminary hearing on the case. His attorney, Mark Geragos, argued that the court should not hold the hearing until a related decision in the California Supreme Court was handed down. But two justices from California’s Second District Court of Appeal rejected the arguments.