Rihanna has reportedly claimed Chris Brown choked her until she was unconscious during an alleged assault on Sunday.

The 'Umbrella' singer is fully co-operating with police — who booked Chris on suspicion of making criminal threats — and has also alleged he threatened to kill her during the attack.

Rihanna and Brown started arguing after leaving Clive Davis' pre-Grammy party on Saturday, resulting in Brown stopping his silver Lamborghini in a Los Angeles side street.

Gossip website E! reports Rihanna threw Brown's car keys out the window, sending the 19-year-old singer into a rage.

After failing to find the keys, he allegedly returned to the car, put his hands around Rihanna's neck and screamed: "I'm going to kill you."

It is claimed Rihanna lost consciousness and when she came round Brown had fled. Officers who attended the scene were said to be so shocked at her "horrific" injuries — including a split lip, bruised face and bite marks — they drove her straight to Cedars-Sinai Medical Centre.

The singer — who is now in hiding — has cancelled her 21st birthday party, which was due to take place on 20 February, and part of her tour.

However, her family have assured her fans she is recovering well.

Her grandmother Clara Brathwaite said: "I don't want people to worry. Rihanna is fine and she is doing well."

Yesterday, the Los Angeles district attorney asked police to look for more evidence in their case against Brown. The investigation needs to be carried out before formal charges are filed against the singer.

District Attorney spokesperson Jane Robinson confirmed: "Prosecutors asked for further investigation. No further comment at this time."

Brown's former step-father has spoken out about the alleged incident, claiming he "wouldn't be surprised" if the singer had assaulted Rihanna.

Donnelle Hawkins — who Brown has accused of abusing his mother, an allegation Donnelle refutes — said: "I wouldn't be surprised. Chris has to take responsibility for his own actions."