Despite Borisov and members of his Citizens for European Development of Bulgaria (GERB) party having been detained during a special operation concerning 120 cases of misuse of European Union funds earlier this year, Bulgaria’s centre-right GERB party has won the country’s parliamentary election, preliminary results show.
Led by ex-Prime Minister Boyko Borissov, GERB took 25.4% of votes with 99% of ballots counted, officials said.
But the 63-year-old faces a difficult task forming a working coalition, as most other parties have already ruled out working with him.
Many of his opponents accuse him of overseeing government corruption during his time in office, which he denies.
Mr Borissov has previously served three separate terms in Bulgaria’s top job, and tackling graft became a key part of an election last November.
Sunday’s poll was called after the centrist government led by Kiril Petkov lost a no-confidence vote in June amid soaring inflation.
The 42-year-old Harvard educated economist ran on an anti-corruption ticket and had urged voters to let him “continue the change” ahead of the vote.
But he conceded defeat on Sunday evening after his We Continue to Change party took just 20.4% of the vote.
Mr Petkov told reporters that “GERB has the responsibility to form a coalition and govern the country”.
He has already ruled out working with Mr Borissov, pledging to be an “opposition scrutinising the spending of every lev”. Pundits say Bulgaria now faces an uncertain political outlook.
“The situation is the same as after the last election but even more complicated,” Daniel Smilov from the Centre for Liberal Strategies, told the BTV network. “Coalitions that were possible then are no longer possible now.”
Mr Borissov says he is willing to work with any party to form a working government, but is likely to have to rely on two pro-Russian parties and the Turkish minority MRF grouping.
“What’s important for us is for reason to prevail,” he said on Sunday. “Believe me, right now I am the most well-meaning and open person to all parties.”
Whoever leads Bulgaria’s next government, the EU’s poorest nation faces a host of challenges.
Inflation has continued to soar – hitting 15% in August – as energy prices spiral amid a dispute with the Russian state-owned gas giant Gazprom.
And ministers will also face a difficult challenge managing the country’s relationship with Moscow, after Mr Petkov’s government turned its back on Vladimir Putin’s regime in April.
Bulgaria is an EU and Nato member, and Mr Borissov recently called President Putin the “clear aggressor” in Russia’s war with Ukraine.
Final results from Sunday’s election are expected to come on Thursday after overseas postal votes are counted.