NSF head Mohamed ElBaradei wrote on his Twitter account that the political group wouldn't participate in "a fake democracy," Ahram Online reported.
The bloc announced Tuesday it would boycott elections that begin April 22, saying the country's election laws were unconstitutional.
Withdrawal of the NSF virtually ensures Islamists such as the Muslim Brotherhood and the ultraconservative Salafist Nour Party will be pitted against each other in the new Parliament, the Los Angeles Times reported.
NSF's appeal, largely limited to Egypt's urban centers, has been unable to match the ability of Islamist groups to attract voters nationwide.
It has "limited options," analyst Magdy Sobhi said, because President Mohamed Morsi has refused to meet key opposition demands that he appoint a new Cabinet and form a committee to revise the Constitution.