Mia Farrow is ending a 12-day fast, and is passing the torch to billionaire Richard Branson.

Farrow had been subsisting on a liquid diet for almost two weeks in order to call the world's attention to the humanitarian crisis in Darfur. Her doctor has ordered her to stop the fast, and Branson will take her place.

Farrow, 64, tells ET: "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 this option.

"When I began this fast twelve days ago," Mia continues, "I hoped another would take my place, and another, and another, until the expelled humanitarian agencies are readmitted and finally there can be justice and peace for the people of Darfur. Richard Branson has stepped forward and so I have ended this fast."

Branson, who is the man behind the "Virgin" brand, will fast for three days, and tells ET: "I'm honored to be taking over the fast for the next three days from Mia Farrow in her courageous stance to support the people of Darfur. Over a year-and-a-half ago, I traveled to Darfur and was horrified by the stories that people of all ages shared with us. … Now, with 13 aid organizations expelled from the country, over one million people are at grave risk. We cannot stand and watch as one million people suffer. We all need to stand up and demand that international aid is restored and that the people of Darfur are protected and given the chance to live in peace."

George Clooney, who has famously supported aid efforts in Darfur, said of Farrow's fast: "I ran into Mia in Chad this January. She'd been living in the camps and villages on the border of Darfur for weeks. She is as dedicated to this cause as any activist I know. It's these kind of efforts that help keep this tragedy in the spotlight."

Many more people around the world have joined in Farrow and Branson's fast. If you'd like you get involved, please visit www.fastdarfur.org.