Nur – detained since January on suspicion of forging documents for registering his Ghad (Tomorrow) party - had begun his hunger strike on Tuesday.
"Dr Nur announced that he had decided to end his hunger strike as soon as he heard of the amendment, which he described as an important step towards the party's and the Egyptian people's demand for extensive constitutional reform," Jamila Ismail, wife of the jailed leader, said.
"He said he highly appreciated this step," she added.
Egyptian President Husni Mubarak on Saturday proposed amending the constitution to let more than one candidate stand in a presidential race, changing the existing system in which parliament chooses a sole candidate to run in a referendum.
Nur's Ghad party had called for many changes to the constitution, but Ismail said one of its key demands was to change article 76, which currently outlines procedures for single-candidate presidential referendums.
Parliament has to approve changes to article 76 and the amendment will then go to a referendum.
The United States has expressed its concern over Nur's detentioin, but the opposition leader said he did not want any foreign intervention.