The party of outgoing Prime Minister Boyko Borisov will not back a possible new cabinet led by the nationalist Patriotic Front coalition.
Support from GERB is crucial for any smaller party or coalition seeking to form a government.
The second-biggest party, socialist BSP will not back a new cabinet either.
Patriotic Front may potentially be entrusted to form a new cabinet if the nationalists were given the opportunity by President Rosen Plevneliev.
Political analysis by Plamen Asenov.