An Iranian diplomat's 20-year prison sentence in Belgium for plotting to bomb an opposition rally outside Paris was confirmed on Wednesday after letting an appeal deadline lapse, prosecutors said. Assadollah Assadi, 49, was convicted in February by an Antwerp court of supplying explosives for the planned June 30, 2018 attack on the exiled opposition group the National Council of Resistance in Iran (NCRI). 

"Mr Assadi's lawyer has failed to lodge an appeal," a spokesman for Belgium's federal prosecutor's office told AFP.

Three individuals sentenced to between 15 and 18 years in prison as accomplices are maintaining their appeals against their convictions, the spokesman said.

Iran has protested Assadi's conviction. Days after the February verdict Iran's foreign ministry summoned Belgium's ambassador in Tehran to convey its fury. 

Belgian police thwarted the 2018 attack when they intercepted a car carrying the bomb, acting on information gathered by several European intelligence services.

Assadi, who was attached to Iran's embassy in Austria at the time, was arrested the following day in Germany, where he was deemed unable to claim diplomatic immunity.

Investigators concluded that he was an Iranian agent working under diplomatic cover.

The trial featured surveillance images of Assadi dressed as a tourist, in a hat and with a camera, handing a couple a package in Luxembourg on June 28, 2018. 

The couple -- Nasimeh Naami and Amir Saadouni, both Iranian-Belgian dual nationals -- were found to have accepted from Assadi a half-kilo of TATP explosives and a detonator.

Another Iranian-Belgian, former dissident Mehrdad Arefani, was also found guilty of being an accomplice, tasked with guiding the couple to the rally.

The NCRI gathering included senior leaders of the dissident group and some high-profile supporters, including former US president Donald Trump's personal lawyer Rudy Giuliani.

There are concerns Iran might seek to pressure Belgium to release Assadi by detaining more European nationals.

An Iranian-Swedish academic who guest lectured at Belgium's VUB university, Ahmadreza Djalali, has been given a death sentence by Iran's regime, which has accused him of espionage.