Lithuania's centre-left opposition has promised a sweeping review of the country's energy policy if it wins Sunday's election, held in tandem with a referendum on building a nuclear plant.

On top in the opinion polls, the Social Democrats say they oppose the current Conservative government's plans to construct a replacement for the Baltic state's lone atomic facility, shut down in 2009.

Despite rising global anti-nuclear sentiments following the 2011 tsunami disaster at Japan's Fukushima plant, Lithuania has pushed ahead with plans involving neighbours Latvia and Estonia.

Japan's Hitachi has been tapped to construct the new plant in northeast Lithuania -- expected to generate 1,350 megawatts from 2020-2022, though final investment decisions are not expected until 2015.

"We disapprove of the current nuclear power plant project," Social Democrat leader Algirdas Butkevicius, the likely new prime minister, told AFP.

Butkevicius said he doubted the project made technical or commercial sense and urged a "No" vote in the referendum.

Under Lithuanian law, the referendum result will be non-binding, but the vote adds to the uncertainty surrounding the project.

"We would not drop the project immediately but would return to an exhaustive analysis," Butkevicius said.

The Social Democrats are expected to form a government with the left-leaning Labour party, running second in the polls.

Labour so far has expressed some support for the nuclear plan, but its leader Viktor Uspaskich sounded a note of caution.

"We will wait until this project is set out, because at the moment neither its cost nor the price of energy are clear," he told AFP.

Closing down the Soviet-era plant -- the same type as the one that exploded in Chernobyl in 1986 -- was a condition of Lithuania's 2004 European Union entry.

But since it provided most of Lithuania's power, the nation of three million is now even more dependent on energy supplies from Russia, with which it has had rocky relations since independence two decades ago.

-- Energy security and Gazprom --

Energy security is a headline issue in Lithuania.

The Social Democrats and Labour have also pledged to seek better ties with Russian energy giant Gazprom.

Gazprom is Lithuania's sole gas supplier, and they are battling over alleged abuse of its market clout to set above-market prices.

Vilnius has sparked a major probe by EU competition authorities and filed a damages claim for almost $1.9 billion with international arbitrators.

"We need to trade with neighbours, not wage war," Butkevicius said.

Gazprom has also faulted Lithuania's moves to implement an EU gas market reform barring suppliers from also running the country's gas distribution system -- a direct challenge to Gazprom which owns 37.1 percent of Lithuania's distributor.

Conservative Prime Minister Andrius Kubilius said the reform, seen as a test case for the whole EU, was crucial to diversify suppliers.

"We cannot have our major pipelines owned by Gazprom," he told AFP, underlining that it would also be easier for Lithuania to build a liquefied natural gas terminal, planned for 2014.

In another vestige of the Soviet past, Lithuania still lacks direct links with Western power grids -- the first is due in 2015.

The Conservatives opposed the nuclear plebiscite from the start, levelling claims of bad faith given that the Social Democrats supported the project when they were in power before 2008.

For Kubilius the project is crucial.

"For the Baltic states, building a new nuclear power station is very important, both from a strategic point of view to have sufficient generation capacity, and also from an economic point of view," he told AFP.

Lithuania's partners have also spoken out.

"It's up to Lithuanians to make this decision either to build it or not. But I would be disappointed if the answer is no," said Estonian Prime Minister Andrus Ansip.

Ironically, a Social Democrat government held a referendum on extending the old plant's life until a new one was ready with the last general election in 2008.

While 89 percent voted in favour, turnout was only 48 percent, rendering the referendum result invalid.

A victory for the "No" side looks likely on Sunday but turnout again seems set to fall short, said Ramunas Vilpisauskas, director of the Institute for International Relations and Political Science in Vilnius.

"Whatever the outcome, there will be various interpretations after the vote. There is a chance the project could be suspended because it has not reached the point of no return," he told AFP.