The Blue Coalition, a grouping of rightist parties, has signalled its willingness to join GERB in government.
"This is a serious victory for the right-wing parties," Petar Moskov, spokesman for the Blue Coalition, said. "It is the big comeback of the rightist parties and a considerable punishing vote for the incumbent coalition."
Before voting got under way, Georgy Parvanov, the president, had urged both GERB and the Socialists to consider entering into a grand right-left coalition for the sake of Bulgaria's stability.
Borisov had campaigned on promises to punish coruption in the European Union's poorest country.
Last year, the European Union cut Bulgaria's access to more than half a billion euros in aid over the country's failure to tackle corruption and organised crime.
On the eve of the election, Bulgarian police arrested at least five people on suspicion of vote buying.
The interior ministry said that it had received more than 100 reports of vote-buying attempts and launched investigations into 15 of the more serious cases.
Media reports said people, especially in poverty-stricken Roma areas, were promised jobs and offered money, chickens, flour, oil, rice, and even drugs to vote.
Despite securing EU membership, the current Socialists-led government has been widely blamed for failing to improve the quality of life in the Balkan country of 7.6 million.
"This means complete, not minor change," Georgi Angelov, an economist at the Open Society Institute in Sofia, said after the exit polls came in.
"It will help restore the trust of the European Union and foreign investors, which will cushion the impact of the global economic crisis."
Although unemployment stands at a relatively low seven per cent, opinion polls suggested more than a third of Bulgarians feared they might lose their job in the near future.
And while wage increases have pushed the average salary to $420, it remains the lowest in the 27-member EU.