Alireza M was found alive in a morgue after being hanged at a jail in the north-eastern city of Bojnord last week.
The condemned man is now being nursed to recovery in preparation for his repeat execution.
“The verdict was the death sentence, and it will be carried out once the man gets well again,” an official said.
Human rights groups believe Iran is second only to China in the number of people it puts to death for crimes ranging from murder and rape to spying and drug-trafficking.
In a separate incident reported by Iranian media on Monday, relatives of a condemned murderer in the western province of Ilam tried to stop his execution at a prison by hurling a grenade.
Around 30 people were injured but the hanging went ahead.
Alireza M, 37, was left to hang for 12 minutes, after which a doctor declared him dead.
But when the prisoner’s family went to collect his body from the prison morgue the next day, they found he was still breathing.
“We found him alive again, which made his two daughters very happy,” an unnamed family member told Iranian state media.
Alireza M was then moved to a hospital where he was being kept under armed guard.
“The horrific prospect of this man facing a second hanging, after having gone through the whole ordeal already once, merely underlines the cruelty and inhumanity of the death penalty,” said Philip Luther, director of Amnesty International’s Middle East and North Africa Programme, in a press release.
Philip Luther called for both a stay of execution for the hanging survivor and a moratorium on all executions in Iran.
According to Amnesty International, at least 508 people may have been executed in Iran this year to date. Most of those killed had been convicted of drug trafficking, it said.
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