Bulgaria is headed for snap elections after the Socialists became the third party to fail to form a government since July elections, returning their mandate on September 7.
Although deadlines are vague under Bulgaria's constitution, the president must now dissolve parliament and call for a fresh vote, which would be the country's third this year.
"Unfortunately, new elections and negotiations await us," President Rumen Radev said.
The Socialists came in third in the vote but got the chance to form a majority government after the failure of the populist There Is Such A People (ITN) and the second-place party of former Prime Minister Boyko Borisov, GERB.
A presidential election is scheduled for November.
"We received no support," said Socialist leader Kornelia Ninova, referring to the three protest parties, who were seen as possible partners but rejected the Socialists.
The ITN won the July 11 elections to take 65 seats, while GERB has 63 seats and the Socialists 36 in the 240-seat parliament.