SOFIA, Bulgaria—Bulgarian Prime Minister Boiko Borisov says his party will stand by its decision not to build a new nuclear power plant after turnout in a voter referendum backing it was too low to be valid.

The Central Electoral Commission said on Tuesday that 60.6 percent of the Bulgarians who voted were in favor of building the plant, but the turnout of 20.2 percent was far below the 60-percent threshold needed to make the referendum binding.

The vote was called by the opposition Socialist party to try to force the center-right government to reverse its decision to cancel a deal with Russia's Atomstroyexport for the plant. The issue has been seen as a barometer of the country's relationship with Russia.

The Socialists say they will revive the plan if they win elections later this year.