* First referendum since the fall of communism in 1989
* Vote a test for PM Borisov ahead of July election
* Voting rules mean referendum unlikely to be binding
Bulgarians are expected to vote in favour of building a new nuclear power plant in their first referendum in the post-communist era on Sunday, challenging the government's decision to abandon the multi-billion-dollar project.
The plebiscite will be seen as a test of public support for the policies of rightist Prime Minister Boiko Borisov ahead of July elections, even though voting rules mean the result of the nuclear referendum is unlikely to be binding.
Borisov, already struggling to revive a lacklustre economy, cancelled the construction of the 2,000-megawatt plant at Belene in March saying the Balkan country could not afford to foot the bill, estimated at more than 10 billion euros ($13.5 billion).
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 opinion polls suggest two thirds of people who say they are going to vote will choose to press on with the plant, many of them hoping it will rein in electricity prices, create jobs and help make the country an energy hub for southeast Europe.
"Bulgaria needs a new nuclear power plant. I do not want my kids to pay high electricity bills and that's what will happen if we give up the construction of the Belene plant," engineer Georgi Avramov, 49, told Reuters ahead of the vote.
Polling stations opened at 6 a.m. (0400 GMT) and will close at 7 p.m. (1700 GMT), with the first exit poll results expected soon afterwards.
While the economy has emerged from a deep recession, it is growing only slowly and many voters are frustrated Bulgaria still trails other ex-communist members of the European Union, with wealth per capita less than half the bloc's average.
Analysts said a strong vote in favour of Belene would be an embarrassment for Borisov, a former bodyguard who has made little progress in his promises to root out corruption since he came to power in 2009.
Unpopular austerity measures imposed by Borisov's ruling centre-right GERB party have already narrowed its lead over opposition Socialists - who called for the referendum - ahead of parliamentary elections in July.
"If we get a strong 'pro-Belene' vote as expected, even though the results are likely to be invalid, we would already have a serious political issue on which Borisov will have to act very carefully," said Gallup International political analyst Kantcho Stoichev.
The result of the referendum, which will ask if a new nuclear power plant should be built and not about the use of nuclear power in general, will only be valid if 4.35 million out of the 6.9 million eligible voters take part - a figure analysts say will be almost impossible to reach.
If 20 percent of voters participate and half of them vote in favour of Belene, the issue will go to parliament for a final decision.
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.
The new plant at Belene, on the River Danube, 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 corruption in the European Union's poorest country.