The star of Titanic and The Aviator did not know the young woman, who gatecrashed the event looking for a former boyfriend in a “highly agitated state” at 4am on Friday.

DiCaprio, 30, was at the party hosted by the Hollywood restaurant-owner Rick Salomon, who made headlines last year when a home-made sex video he made with Paris Hilton, the hotel heiress, appeared on the internet.

Ken Sunshine, DiCaprio’s spokesman, said the actor was in Los Angeles during a short break from filming The Departed. “While leaving a small private gathering Leo was attacked by a woman who was trespassing and had been repeatedly asked to leave the property,” he said.

“The attacker stuck him with a glass object before being restrained by witnesses. She was reportedly looking for an ex-boyfriend who she had apparently physically assaulted on prior occasions,” said Sunshine.

According to witnesses another guest, Jonah Blechman, who acted alongside DiCaprio in the film This Boy’s Life, shouted out: “My God, she has slashed open his face.”

Blechman and another friend propped up the star, who was in shock as blood streamed from his head, bundled him into a car and drove him out of the Hollywood Hills to the nearby Cedars-Sinai hospital for emergency treatment.

Blechman then drove DiCaprio back to the star’s home in the Hollywood Hills above Sunset Strip, where neighbours include other members of his party bratpack including Tobey Maguire and DiCaprio’s girlfriend Gisele Bundchen, the Brazilian supermodel.

“Leo’s been attacked. She smashed his face in and we rushed to help,” Blechman told the News of the World.

A close friend told the paper: “He’s obviously still very shocked over what happened to him. One minute he was enjoying the party and the next he was hit in the head. We can’t believe what’s happened. He’s recovering but he’s still in pain.”

The attack could have been more serious. Victor Storey, a doctor from Havering in Essex, said: “It sounds like Leonardo’s a very lucky man indeed. If the bottle had severed the main artery in the side of the neck — the carotid artery — he’d have been dead in seconds.

“It’s almost impossible to staunch an injury like that. And just beside it is the jugular vein. If that gets cut you have to have immediate treatment otherwise you’ll just black out and die within minutes from loss of blood.

“Twelve stitches could well cover up to 2½ inches of Leonardo’s face and shows that he must have been hit with some force.”

DiCaprio tries to avoid physical confrontations, not just to protect his boyish face, his most important asset as a film star, but also following a tangled lawsuit five years ago. Then it was alleged that he became involved in a street brawl with a scriptwriter who complained he had made advances in his girlfriend.

The scriptwriter, Roger Wilson, claimed DiCaprio had asked his friends to assault him. Wilson sued the actor, claiming a blow on his throat had broken his career as a singer. A judge dismissed the case.

The following year DiCaprio was involved in another brawl, in London during a pop launch at Soho’s Chinawhite club. Since then DiCaprio has become more active in environmental causes and has said he loathes all violence.

The assault is not expected to affect DiCaprio’s return to work on The Departed for director Martin Scorsese. Los Angeles police are investigating the crime, said Sunshine, but no arrests had so far been made.

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