"I will hand in my resignation tomorrow or the day after... The results clearly show that the ruling coalition no longer holds the majority," the centre-right premier told reporters today evening.
The announcement is likely to trigger early elections by next spring and risks plunging Bulgaria into renewed political turmoil just two years after Borisov took office for the second time.
It came shortly after projections showed that ex-airforce chief and political novice Rumen Radev had swept close to 60 percent of ballots, while ex-parliament speaker Tsetska Tsacheva obtained just over 35 percent.
Nearby Moldova also looked set to elect a pro-Russian president today.
"It's a victory for all Bulgarian people. Democracy has beaten apathy and fear today," Radev told public broadcaster BNT today evening.
Radev's clear support for the lifting of EU sanctions on Russia over Ukraine and ambivalent statements about the EU and NATO have prompted analysts to speculate that he could pursue closer ties with Moscow.
The straight-laced Tsacheva meanwhile has failed to sway disgruntled voters seeking to punish the government over its perceived failure to tackle rampant corruption and poverty in the European Union's poorest member state.
The Bulgarian president's role is largely ceremonial but the incumbent is nonetheless a respected figure and commander-in-chief of the armed forces.
Borisov appears to have badly miscalculated in nominating Tsacheva, analysts say.
"His threat to step down has mobilised his opponents more than supporters," Zhivko Georgiev of the Gallup Institute told AFP.
Disclaimer: No Business Standard Journalist was involved in creation of this content