Former Egyptian president Mohamed Morsi, imprisoned since the military removed him from office in 2013, has died after suddenly collapsing during a court session.

The 67-year-old died Tuesday (Australian time) while facing a trial for allegedly breaking out prisoners during the 2011 protests that toppled longtime autocrat Hosni Mubarak.

Morsi, a top figure in Egypt’s largest Islamist group – the now outlawed Muslim Brotherhood – reportedly fainted minutes after warning that he could reveal “many secrets”, an unnamed judicial official said.

He had been giving an address from behind the glass cage that prisoners are kept in during hearings.

In a Facebook post, Morsi’s son, Ahmed, confirmed the death of his father.

“In front of Allah, my father and we shall unite,” he wrote.

Morsi was serving a 20-year jail sentence on a conviction related to the deaths of protesters during 2012 demonstrations, and a concurrent life sentence for espionage in a case related to Qatar.

He has been in prison undergoing multiple trials ever since the military ousted him in July 2013 and launched a massive crackdown on Muslim Brotherhood.

Tuesday’s session was part of a retrial, being held inside Cairo’s Tura Prison, on charges of espionage with the Palestinian resistance movement Hamas.

Leading member of the Muslim Brotherhood in London Mohammed Sudan described Morsi’s death as “premediated murder”, saying the former president was banned from receiving medicine or visits and there was little information about his health condition.

“He has been placed behind glass cage (during trials). No one can hear him or know what is happening to him,” he said.

“He hasn’t received any visits for a months or nearly a year. He complained before that he doesn’t get his medicine. This is premeditated murder. This is slow death.”

The judicial official said Morsi had asked to speak to the court during the session.

The judge permitted it, and Morsi gave a speech saying he had “many secrets” that, if he told them, he would be released.

He added that he wasn’t telling them because it would harm Egypt’s national security.

A spokesman for the Interior Ministry did not answer calls seeking comment.

Morsi was being held in a special wing in the sprawling Tora detention complex nicknamed Scorpion Prison. Rights groups say its poor conditions fall far below Egyptian and international standards.

-with AAP