Al-Wasat Party announced on Tuesday that it is “initially boycotting” the constitutional referendum set to take place in mid-January.
The party released a statement saying that this is an initial decision and that there would be further coordination “to reach a decision” with the Anti-Coup Alliance, also known as the National Coalition to Support Legitimacy.
The alliance had announced one day earlier that it would boycott the referendum.
Constituent Assembly head Amr Moussa had called on Sunday for all sectors of Egyptian society to participate.
Al-Wasat Party is one of the backers of the alliance, which is made up of several parties and organisations including the Muslim Brotherhood.
Like the alliance, Al-Wasat Party has opposed the ouster of former President Mohamed Morsi on 3 July, referring to it as a “coup.”
Top party officials Aboul Ela Mady, who served as chairman of the party, and Essam Sultan, who served as the deputy head, were arrested together and have been in detention for 141 days.
Their arrest is part of a larger crackdown, which began after Morsi’s ouster, led by authorities targeting prominent Islamists.
Interim president Adly Mansour announced on Saturday that the referendum for the amended 2012 constitution is to take place over two days, on 14 and 15 January.
Several parties have announced their positions on the referendum throughout the month, including Al-Dostour Party which called last week on Egyptians to “strongly participate”. It also expressed its rejection of any campaign calling for the referendum’s boycott.
The Salafi Al-Nour Party announced they would vote Yes while moderate Islamist party Misr Al Qawia Party announced its position last week, urging Egyptians to vote No.