Ayman Nour is refusing to eat in protest at being detained on forgery charges, his wife said on Saturday.
Nour, who came a distant runner up in September's presidential election, is a diabetic, and doctors said the hunger strike could lead to a coma and eventually death as his blood becomes more acidic, damaging his vital organs.
Gameela Ismail, Nour's wife and spokeswoman, said: "He has been moved to the prison hospital, suffering from a high level of acetone in his blood and urine, which eventually leads to a coma and threatens his life."
Nour was imprisoned on 5 December for the duration of his trial, along with six others, on accusations they forged the papers required for his party to be officially recognised last year. The verdict is due on 24 December.
The United States, which has called for greater political freedom in Egypt, says it is watching the trial closely. Nour was first held for questioning in January, for six weeks, and was released in March when the US put pressure on Cairo.
Nour, a 40-year-old lawyer, emerged as the strongest of eight candidates who challenged incumbent Hosni Mubarak in September's presidential election. He mustered 7.6% of the vote against the veteran president's 88.6%.
Nour has suffered politically
since the presidential election
However, since then he has lost his parliamentary seat in Cairo and his Ghad party has been plagued by internal dissent that has been exacerbated by his forgery trial.
Ismail said doctors were insisting they correct the imbalance in Nour's blood with fluids administered intravenously. She said he intended to continue to refuse to eat, but is considering the fluid replacement.
Nour says his trial is politically motivated, an allegation the government denies.