An Iranian diplomat has been convicted of masterminding a thwarted bomb attack against an exiled Iranian opposition group in France in 2018.

Assadollah Assadi was sentenced to 20 years in prison by a Belgian court that rejected his claim of diplomatic immunity.

Assadi, a Vienna-based diplomat who was arrested in Belgium refused to testify during his trial last year, invoking his diplomatic status.

He did not attend Thursday’s hearing at the Antwerp courthouse.

Prosecutors had requested the maximum prison sentence of 20 years on charges of attempted terrorist murder and participation in the activities of a terrorist group. Assadi contested all the charges against him.

Three other defendants also received jail sentences.

During the trial, lawyers for the plaintiffs and representatives of the Mujahedeen-e-Khalq opposition group, or MEK, claimed without offering evidence that the diplomat set up the attack on direct orders from Iran’s highest authorities. Tehran has denied having a hand in the plot.

Assadi’s conviction comes at a critical time and has the potential to embarrass his country as US President Joe Biden’s administration weighs up whether to rejoin the 2015 nuclear deal between Tehran and world powers.

Iran also said last month it expects Washington to lift economic sanctions that former president Donald Trump imposed on the country after pulling America out of the deal in 2018.

On June 30 2018, Belgian police officers, tipped off by intelligence services about a possible attack against the annual meeting of the MEK, stopped a couple travelling in a Mercedes car. In their luggage, they found 550 grams of the unstable TATP explosive and a detonator.

Belgium’s bomb disposal unit said the device was of professional quality. It could have caused a sizable explosion and panic in the crowd, estimated at 25,000 people, that had gathered that day in the French town of Villepinte, north of Paris.

Among dozens of prominent guests at the rally that day were Mr Trump’s lawyer, Rudy Giuliani; Newt Gingrich, former conservative speaker of the US House of Representatives; and former Colombian presidential candidate Ingrid Betancourt.

Assadi was arrested a day later in Germany and transferred to Belgium. The court said that since Assadi was on holiday at the time of his arrest he was not entitled to immunity.

A note from Belgium’s intelligence and security agency seen by The Associated Press identified him as an officer of Iran’s intelligence and security ministry who operated under cover at Iran’s embassy in Vienna.

Belgium’s state security officers said he worked for the ministry’s so-called Department 312, the directorate for internal security, which is on the European Union’s list of organisations regarded as terrorist.

Prosecutors said he was the “operational commander” of the attack and accused him of recruiting the couple — Amir Saadouni and Nasimeh Naami — years before the attack, to obtain information about the opposition. Both were of Iranian heritage.

Saadouni was sentenced to 15 years in jail while Naami was handed a 18-year sentence.

According to the investigation, Assadi carried the explosives to Austria on a commercial flight from Iran and later handed the bomb over to the pair during a meeting at a Pizza Hut restaurant in Luxembourg.

The fourth defendant, Mehrdad Arefani, was sentenced to 17 years in prison.

The MEK, once an armed organisation with a base in Iraq, is the most structured among exiled Iranian opposition groups, and is detested by Iranian authorities.

It was removed from EU and US terrorism lists several years ago after denouncing violence and getting western politicians to lobby on its behalf. The MEK supports a hard line on Iran and backs US sanctions on the country.