President Rumen Radev handed the mandate to Borisov, calling on him to address "the most pressing expectations of Bulgarians for (improved) security, incomes, justice and rule of law,"
The conservative GERB party of the 57-year-old Borisov won the March 26 snap general elections after his previous right-wing coalition cabinet resigned in November.
But with 95 lawmakers GERB found itself unable to form a governing majority in the 240-seat parliament.
After weeks of negotiations, GERB signed earlier today a coalition agreement with a small three-party ultra-nationalist alliance, the United Patriots (UP), with 27 lawmakers.
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Despite its strong anti-migrant talk and calls to reform the European Union to take into account the national interests of its members, the UP have never questioned Bulgaria's EU and NATO membership and the agreement reiterated that the two organisations are "a guarantee for the security of the country."
"The Bulgarian government will work for the full-fledged participation of the country in the Euroatlantic structures while standing up for Bulgaria's national interest," it added.
They did not provide any further details but UP was largely expected to take several ministerial posts.
Borisov is expected to announce the full line-up of his cabinet next yesterday, a day before it takes the oath of office in parliament.