Nur's wife, Jamila Ismail, said on Thursday said that the al-Ghad (Tomorrow) Party had not eaten since Tuesday evening and told a driver who visited him on Thursday that he refused to eat.
"The driver said he was in very bad health and in a very bad mood ... he was saying: 'Where are you, everybody?'" she said.
The driver found six or seven policemen standing around him trying to persuade him to eat but Nur told the driver that he wanted only newspapers, cigarettes and diabetes strips to test his blood-sugar level.
Nur had already sent back the food that the driver took for him on Wednesday, she added.
Nur was detained in January for questioning about allegations that his party submitted forged documents in its application for recognition last year. He has not been charged.
His supporters said the allegations are fabricated to punish Nur for advocating radical political change.
He has been one of the most vocal advocates of constitutional changes that would make it more difficult for President Husni Mubarak to obtain a fifth six-year term in office when his current term expires in October.
Nur was taken to the prison hospital on Tuesday after he fell ill during interrogation in the middle of the night but he went back to his prison cell on Wednesday.
The United States has said it has "very strong concerns" about the Nur case, but the detained leader has said he has not asked for and does not want any foreign intervention.