Bulgaria's Socialists, allies in the outgoing coalition government of Prime Minister Kiril Petkov, said on Wednesday they would not back a new Petkov-led Cabinet, angry with the expulsion of 70 Russian diplomats.
The Socialists' position could bring the European Union and NATO member state closer in the autumn to holding fresh elections, the fourth since April 2021.
The Russia-friendly Socialists called the expulsion of the diplomats over suspected espionage unacceptable.
"We are putting on hold any talks with the PP party for forming a new government," Socialist leader and outgoing deputy premier Kornelia Ninova said.
"The decision to expel 70 Russian diplomats is unprecedented ... For us it is completely unacceptable," she said.
Ninova said Petkov has made the decision to expel the Russians without government approval and that her party could return to the negotiating table for a new coalition government with PP if Petkov is not nominated as prime minister.
President Rumen Radev, at odds with the outgoing reformist premier, declined to comment on the expulsions, saying he lacked detailed information, but expressed hope that Petkov had considered the consequences.
Speaking ahead of a closed hearing at the parliament on the expulsions, Petkov said the decision was taken following a sitting of the country's security council on June 9 and three reports from the intelligence services from May and June.
"There is some kind of misunderstanding. The decision of what happened is not mine," Petkov told reporters.
His ruling PP party is trying to muster a majority for a new government after his Cabinet was ousted in a no-confidence motion last week, but analysts were sceptical he could succeed, even before the Socialists' decision.
Following the invasion of Ukraine, Petkov has taken an unusually strong stance against Russia for a country that has cultural, religious and historical ties with Russia and long been a draw for Russian tourists.
He did not comment on the Socialists' position.
(Reporting by Tsvetelia Tsolova in Sofia; Editing by Nick Macfie and Matthew Lewis)
