The Court of Cassation's ruling means Mohamed Morsi will be given a new trial, alongside five other leaders of his now-banned Muslim Brotherhood group, whose death sentences in the same case were also quashed.

The court also struck down life sentences passed in the same case against 21 Brotherhood members.

Last month, a court upheld a 20-year sentence for Morsi on charges arising from the killing of protesters in December 2012. Morsi was ousted by the military in 2013 after just one year in office.

Morsi, Egypt's first freely elected president, is implicated in four lengthy trials pertaining to different cases against him, including the 2011 prison break during the uprising against his predecessor, Egyptian autocrat Hosni Mubarak.

For the prison break, Morsi had received the only death sentence so far.

He is also appealing against life sentences he received in the two remaining cases against him - one on espionage charges related to Qatar and the other on suspicion of conspiring with the militant Palestinian Hamas group that runs the Gaza Strip. Those appeal proceedings are pending.

Morsi's son Osama, who is also one of his defence lawyers, welcomed the cancellation of the death sentence. "I am optimistic but I also don't expect much from politicised judiciary," he said.

He said his father has been in solitary confinement since 2013 and has been banned from any family visits or meetings with lawyers.

"He lives in complete isolation," Osama Morsi said.