Mel Gibson's pregnant girlfriend has already slapped him with an ultimatum: "Go to rehab - or we're through!"

The ENQUIRER has learned Russian beauty Oksana Grigorieva - the woman behind Mel's $1 billion divorce - is demanding the Oscar winner get professional help with his booze addiction or she's prepared to cut all ties.

"Oksana wants Mel to deal with his drinking on a long-term basis," a source close to the couple told The ENQUIRER.

"Mel's got a hair-trigger temper when he's drunk, and Oksana insists she won't stick around when he's like that. She's informed Mel that if he doesn't go to rehab, she'll bow out of the picture before they go any further."

The 53-year-old superstar - whose 2006 DUI arrest and anti-Semitic meltdown made headlines around the world - has struggled for years with a drinking problem. And while he's owned up to alcoholism and is a big AA supporter, the troubled Braveheart star has never entered a residential rehab program to deal with his addiction.

His long-suffering wife Robyn slapped Mel with a divorce suit on April 9 after 28 years of marriage. And Oksana - who is four months pregnant with his baby - believes alcoholism is the reason the hunky thesp indulged in so many extramarital affairs, according to another source.

"Oksana thinks Mel will be less likely to cheat on her if he's not out drinking," a Gibson family friend told The ENQUIRER.

"But she knows that Mel's very stubborn."

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