Bulgaria's largest party, the center-right GERB, expects to form a government with three nationalist parties by late April and return Boiko Boriskov to power as prime minister, a senior party official said on Monday.
Borisov's GERB won 95 seats in the general election on March 26, beating its leftist Socialist rivals, but it failed to gain an outright 121-seat majority in parliament.
His resignation late last year triggered the early election.
GERB has told the third-placed United Patriots (UP), a nationalist alliance of three parties, that the prime minister's post will not be subject of their coalition negotiations.
"The prime minister of the next government that will be formed I suppose by the end of this month ... will be Boiko Borisov", said Vladislav Goranov, an MP and member of GERB's political negotiating team. "There's no doubt about that."
Goranov's comments to reporters came after one of the nationalist leaders had suggested that Borisov, 57, should not lead the next government.
Borisov quit as premier after a GERB-backed candidate lost a presidential election in November to Rumen Radev, a Russia-friendly ally of the Socialists. Bulgaria is currently being run by a caretaker administration.
The UP alliance campaigned to boost low living standards and double the minimum monthly state pensions, now at 160 levs ($87.25) - the lowest in the European Union.
Analysts say such demands, coupled with GERB's plans to double teachers' wages within four years, may boost public spending and pose risks to Bulgaria's currency peg to the euro.
But Goranov, a former finance minister likely to get the same post in the next government, told reporters he was not worried for state coffers.
A coalition with the nationalists would have just one seat above the majority threshold of 121 seats and Goranov said GERB would also seek support of smaller, populist grouping of businessman Veselin Mareshki.
Bulgaria's polls suggested the country would continue with its fiscal and economic policies but was unlikely to break a pattern of unstable governments that have hindered structural reforms, Fitch rating agency said last week.
The timing for inter-party negotiations has yet to be set.
GERB has not ruled out leading a minority government, but Goranov called such an option "extreme."
(1$= 1.8338 leva)