Ammon News - Cairo (Reuters) - Mohamed ElBaradei, Egypt's former interim vice president, is being sued for a "betrayal of trust" over his decision to quit the army-backed government in protest at its bloody crackdown on the Muslim Brotherhood.

The case points to the prospect of a new wave of politically driven lawsuits being brought to court following the downfall of President Mohamed Mursi, whose supporters brought a raft of cases against opposition figures during his year in power.

Anti-government activists had called those suits, many of them accusing people of "insulting the president", a form of political intimidation.

ElBaradei's case, brought by an Egyptian law professor, will be heard in a Cairo court on September 19, judicial sources said on Tuesday.

ElBaradei, the former head of the U.N. nuclear agency and co-leader of the secular National Salvation Front (NSF) grouping, was the most prominent liberal to endorse the military's overthrow of Mursi on July 3 following mass protests.

But he resigned on August 14 after security forces attacked the protest camps set up by Mursi's Muslim Brotherhood supporters in Cairo, killing hundreds of people.

The military's intervention against Mursi has polarized public opinion in Egypt. Around 900 people have died in violence across the country over the past week.

Sayyed Ateeq, a law professor at Helwan University, filed the suit against ElBaradei.

"He was appointed in his capacity as a representative of the NSF and the majority of the people who signed the Tamarod declaration," he told Reuters, referring to the broad movement that led the anti-Mursi protests.

"Doctor ElBaradei was entrusted with this position and he had a duty to go back to those who entrusted him and ask to resign" instead of stepping down on his own, he said.

Ateeq said that, if found guilty, ElBaradei could face a three-year prison sentence. But a judicial source said the maximum sentence in a case of this kind was a fine and a suspended jail term.

ElBaradei left Egypt this week for Europe and is unlikely to attend any hearing in the case.

Khaled Dawoud, an aide to ElBaradei who quit as NSF spokesman following the crackdown, said Ateeq "set a precedent that harms Egypt's reputation abroad, when a politician is prosecuted just for resigning from his post, something that has never happened before in any country in the world".

The lawsuit follows a wave of arrests of Muslim Brotherhood leaders in recent days and a decision by the public prosecutor to charge Mursi, who is being detained in an undisclosed location, with inciting violence.

"If this case against ElBaradei is true then it is a major escalation showing that things are getting very polarized. You're either on this side or on that side," Dawoud told Reuters.