At least eight deputies formally resigned from the Islamist-dominated Shura Council, Egypt's traditionally toothless upper house which took over legislative duties after parliament dissolved last year.
Several others had tendered their resignations but they are yet to be accepted, upper house speaker Ahmed Fahmy said in a statement.
The resignations come as Egypt is deeply divided between Mursi's mainly Islamist supporters and a broad-based opposition.
The president is accused of betraying the 2011 revolution that brought him to power, concentrating power in the hands of Islamists and failing to manage the country's affairs.
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Ihab al-Kharatt, who heads the human rights council at the Shura Council, said at least 22 deputies had quit.
"We resigned in support of 22 million Egyptians who withdrew their confidence from Mursi," Kharatt told AFP.
He was referring to a petition launched by grassroots movement Tamarod (Arabic for Rebellion) which says it collected 22 million signatures to demand Morsi's departure and a snap presidential election.
This figure is higher than the number of people who voted for Mursi in last year's presidential election -- 13.23 million, or 51.7 per cent of the ballots cast.