Egyptians vote on Wednesday on amendments to Article 76 of the constitution that will allow multiple candidates to stand in the presidential elections.
But on Saturday, Nur said the party leadership had decided to boycott the referendum. Nur is the only opposition leader yet to declare his intention to contest the upcoming presidential election due to be convened in September.
Nur said his party had preferred that voters be asked not only to say "yes" or "no" to the proposed changes, but also be given a chance to reject conditions in the text without refusing the principle of change.
"There should have been a third question," he said.
The opposition parties are not opposed to the principle of amending the constitution, but rather to conditions in the text that prevent them from mounting any meaningful challenge to the ruling party's candidate.
Al-Ghad became the fourth opposition party to urge supporters to stay at home on referendum day after the leftist Tagammu, Nasserist, centre-liberal Al-Wafd parties, in addition to the banned but tolerated Muslim Brotherhood.
The opposition, both secular and Islamist, charges that the amendment sets such tight conditions for registration that only the ruling party will be able to field a candidate, undermining the whole point of the reform.