Iran's former President Mahmoud Ahmadinejad on Wednesday registered to run in the presidential race, to be held next month, defying an edict from the country’s supreme leader not to do so.

Reporters with the 'Associated Press' described watching as stunned election officials processed the hardliner’s paperwork.

Ahmadinejad's surprise decision will likely upend an election many believed would be won by the incumbent, Hassan Rouhani, who negotiated the nuclear deal with world powers.

It also makes Ahmadinejad the most high-profile Iranian politician to defy the wishes of Supreme Leader Ayatollah Ali Khamenei, who has the final say in all state matters. Last year, Khamenei took the unusual decision to reveal he had told Ahmadinejad he did not recommend he enter the contest.

But in a news conference shortly after submitting his registration, Ahmadinejad described the comments by the supreme leader as “merely advice”. 

Ahmadinejad had initially said in a press conference last week that he was formally backing a former deputy, Hamid Baghaei, and that he did not plan to run himself. 

He previously served two four-year terms from 2005 to 2013. Under Iranian law, he became eligible to run again after four years out of office, but he remains a polarizing figure, even among fellow hardliners. He was notorious for his repeated questioning of the Holocaust, his vicious attacks against Israel and the United States, and his hard-line stance against regime critics.

Iran’s economy suffered under heavy international sanctions during his administration because of western suspicions that Tehran was secretly pursuing nuclear weapons.