"The trip is not going to happen," Hossein Sheikholeslam, secretary general of the International Conference for the Support of the "Palestinian" Intifada, an Iranian body set up by parliament, told reporters on Thursday, IRNA said.
He said the ship had originally been due to depart on Thursday, but "due to restrictions from the occupying Zionist regime, it was decided that this ship would leave on Sunday. But now the trip is not going to happen."
Sheikholeslam, speaking in the northern Iranian city of Rasht, said that the aid supplies that had been collected for the voyage will be sent by other means to Gaza.
"The Zionist regime has made the blockade a political issue and we do not wish to politicise this kind of humanitarian aid because the most important thing for us is to break the blockade of Gaza," he said.
He said the voyage was cancelled as Israel "had sent a letter to the United Nations saying that the presence of Iranian and Lebanese ships in the Gaza area will be considered a declaration of war on that regime and it will confront it," IRNA quoted him as saying.
"In order to deprive the Zionist regime of any excuse, the aid collected for the oppressed people of Gaza will be delivered to them by other means without mentioning the name of Iran."
On Tuesday, Iranian Red Crescent official Abdolrauf Adibzadeh had said that the ship, "Gaza Children", would leave on Sunday from the Gulf port of Bandar Abbas.
It was to have carried 1,100 tonnes of aid including medicine and foodstuffs, he said.
The Iranian Red Crescent had initially planned to send two aid ships to the Palestinian territory earlier this month.
But an official with the organisation said on Monday the departure for Gaza had been delayed because of a lack of coordination and a change of cargo.
Iran's decision to send aid came after a May 31 Israeli commando raid on a Gaza-bound aid flotilla left nine
The Islamic republic does not recognise Israel, and tensions have worsened between the two countries under President Mahmoud Ahmadinejad who has repeatedly said the Jewish state is doomed to vanish.
Israel, the sole if undeclared nuclear-armed country in the Middle East, accuses Iran of seeking nuclear weapons under the guise of a civilian programme and has not ruled out a military strike against Tehran's atomic drive.
Iran denies the charge.