* Vote seen as big test for government before July election
* First referendum since the fall of communism in 1989
* Voters want nuclear plant, low turnout invalidates result
By Tsvetelia Tsolova
SOFIA, Jan 27 (Reuters) - Bulgarians backed the building of a nuclear power plant in a referendum on Sunday, a blow to Prime Minister Boiko Borisov who had abandoned the multi-billion-dollar project, but turnout was not enough to overturn the government's decision.
Roughly 60 percent voted for a 2,000-megawatt plant at Belene on the Danube River, while 40 percent opposed it, Gallup International and Alpha Research exit polls showed. Turnout was about 20 percent.
The country's first referendum since the fall of communism, showed that Borisov's policy has alienated many voters in the European Union's poorest country and complicated his campaign for a July election, analysts said.
Exit polls put the turnout between 20.2 and 21.8 percent, well below the required 60 percent to make the vote binding.
But under the voting rules, if more than 20 percent of eligible voters took part and more than half of them voted in favour, the issue will be sent to parliament, where Borisov's GERB party has a working majority, for a final decision.
Borisov said that if the turnout is enough for Belene to go before parliament, GERB will reject the project again.
"The result clearly puts pressure on the government. The low turnout however indicates there are no clear winners and we will be facing a serious stalemate after the July election," said Kiril Avramov, political analyst with New Bulgarian University.
Borisov, struggling to revive the economy, cancelled the construction of Belene in March saying the Balkan country could not afford estimated costs of more than 10 billion euros ($13.5 billion), and after failing to attract Western investors.
Bulgaria's allies in Brussels and Washington also opposed the project, fearing it would deepen the country's economic and political dependence on Russia - Moscow had offered to finance the plant which would have been built by its Atomstroyexport.
But many hoped it would rein in electricity prices, create jobs and help make Bulgaria an energy hub for southeast Europe.
While the economy has emerged from a deep recession, it is growing only slowly and many voters are frustrated that Bulgaria still trails other former communist members of the European Union, with wealth per capita less than half the bloc's average.
Analysts said the vote in favour of Belene comes as a blow to Borisov, who has made little progress in his promises to root out corruption since he came to power in 2009.
Unpopular austerity measures imposed by the ruling centre-right GERB party have already narrowed its lead over opposition Socialists, who called for the referendum, ahead of parliamentary elections in July.
Speaking to reporters after casting a ballot, Borisov played down the link between support for the plant and the July elections, stressing that the low turnout showed Bulgarians were not interested in the issue.
"I want to thank (Socialist Party leader Sergei) Stanishev. By organising this referendum he put a final cross on the Belene project," he said.
The result of the referendum, which asked if a new nuclear power plant should be built and not about the use of nuclear power in general, can only be valid if 4.35 million out of 6.9 million eligible voters take part.
Bulgaria has an operational 2,000 megawatt nuclear power plant at Kozloduy and has hired U.S. firm Westinghouse to draw up plans to add another 1,000 MW unit at the site.
Belene was also opposed by environmentalists, who said it would be built near an earthquake-prone area, and by rights groups who said the high-cost project would encourage graft.