Angelina Jolie is battling weakness fatigue as she tries to lose 21lbs in 21 days for eagerly-anticipated new film.

Squealing spies with Grazia Magazine say the 33-year-old mother of six is desperate to slim down for her role in the upcoming action-thriller Salt, in which she plays a CIA assassin who is forced to disguise herself as a man.

Angelina is so determined to look like a boy she has turned to the Martha’s Vineyard Detox Diet to help shrink her already thin frame. The crash diet, which gained popularity in Hollywood in 2007, encourages followers to ingest liquids every two hours to “cleanse the body,” but ingesting Solid food is strictly prohibited.

Friends say the Oscar-winning star began her detox three days before the 2009 Academy Awards last month.

“She has been trying to go for the full 21 days but it’s tough. She’s lost around eight pounds but is now introducing a couple of small meals in between shakes as she was starting to feel weak,” an insider says.