Rihanna in b-day food fight.

Rihanna celebrated her 20th birthday with a cupcake fight.

The Umbrella singer – who doesn’t turn 20 until next Wednesday – was partying with friends including Kanye West and actor Wilmer Valderrama at Los Angeles’ Les Deux nightclub when the food fight began on Tuesday night.

An onlooker said: “There was cupcake frosting all over Rihanna’s face and the group was hysterically laughing to the point of almost tears!”

Chris Brown, who is rumoured to be dating Rihanna, was later given a standing ovation after he sang her an acapella version of Happy Birthday.

Despite Chris insisting he and Rihanna are “just friends”, onlookers at the exclusive club said the pair were “cuddling and acting very cosy all night”.

The couple also attended the Grammys together last weekend and stocked up on goody bags. Rihanna picked up Balenciaga sunglasses, while 18-year-old Chris took home two pairs of Marc Jacobs sunglasses in red and blue.

When they realised they might be photographed together, Chris insisted they should leave.

A Grammys source said: “Chris was telling her they needed to go, and in seconds they were gone.”