The judges ruled that the four could not be tried in absentia because prosecutors had been unable to officially inform them of the judicial process against them, a court-appointed defence lawyer told AFP.

The officers stand accused of kidnapping, conspiracy to murder and grievous bodily harm in the case, which sparked outrage in Italy and has strained diplomatic relations with Egypt.

Regeni's parents and sister had been present at the hearing in the bunker room of Rebibbia prison, often the stage for mafia trials.

The 28-year-old was doing research for a doctorate at Cambridge University when he was abducted in Cairo in January 2016.

His body, bearing extensive signs of torture, was eventually found dumped on the outskirts of Cairo ten days later, naked from the waist down.

The Italian government joined the proceedings with a civil suit for damages, in a symbolic show of support.

But before the trial could begin, the court had first to rule on whether the four suspects, members of Egypt's National Security Apparatus (NSA), were aware of the proceedings against them. Egypt has repeatedly refused to provide their contact details.

Since the death of Regeni, the Egyptian prosecution has been conducting an investigation into the case for years while cooperating with its Italian counterpart, but has eventually stated that so far the perpetrators have not been identified.

In January, Egypt’s Public Prosecution said it handed documents to the Italian side that refute alleged involvement of Egyptian officers in the disappearance and murder of Regeni amid the ongoing trial at the time in Rome against the suspected security members.

Earlier in December, the Egyptian prosecution said it will temporarily not pursue a criminal lawsuit in the case due to the lack of knowledge of the perpetrator of the crime.

It also announced at the time that it was ruling out from the case the accusations raised by Italy's prosecutor-general against the police officers in Egypt’s NSA of involvement in the murder.

Egypt’s and Italy’s prosecutions have published a joint statement on 30 November to give their final findings on the case. Egypt's public prosecution said the perpetrator of the murder of the Italian student “is still unknown,” and it accordingly “will temporarily close its investigation,” while instructing investigative authorities to continue taking all necessary measures to reach the perpetrator of the crime.

At a preliminary hearing in May, an Italian judge ruled that media coverage meant news of the trial would have reached them.

But the court overturned that ruling on Thursday, effectively sending the prosecution back to square one.

*This story was edited by Ahram Online