The actress began a hunger strike on April 27 and hoped to survive without food for 21 days as part of an effort to raise awareness about the plight of starving refugees in the Sudan.
Earlier this week, she revealed she felt fine, but medics monitoring her health have advised her to stop fasting.
In one recent blog posting, the Rosemary's Baby star wrote, "Feeling awful. Blood sugar under 40. Muscles hurt. I won't be able to continue much longer."
And in her latest online message to fans, she adds, "I'm really struggling today. I have been instructed by my doctor to stop my fast immediately due to health concerns - including possible seizures.
"I am fortunate. The women, children, and men I am fasting for do not have that option.
"When beginning this fast twelve days ago, I said that when I could go no longer, I hoped another would take my place, and another, and another, until the expelled humanitarian agencies are readmitted and finally there is finally justice and peace for the people of Darfur."
According to the blog, Virgin boss Richard Branson has "stepped forward" to take on the celebrity fasting chain.
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