The first reactor at the Ignalina plant -- of the same design as Ukraine's Chernobyl facility, site of the world's worst nuclear disaster in 1986 -- was shut down in late 2004.

The remaining reactor is due to be closed at the end of next year, as agreed with the EU during Lithuania's accession negotiations. Lithuania joined the 27-nation bloc in 2004.

Analysts have warned that electricity prices could double after Ignalina's shutdown, fuelling already double-digit inflation in Lithuania and slashing economic growth rates.

The Baltic state is also concerned its energy dependence on Russia will increase, tying the ex-Soviet republic -- now an EU and NATO member -- even more tightly to its former master.

Voters will be asked whether they would agree to extend Ignalina's lifespan by "technically safe terms", the press service said, without elaborating.

"By technically safe terms we meant 2012, when the reactor's fuel channels will have to be replaced," Birute Vesaite, head of the parliamentary economics committee, told Reuters.

Lawmakers ignored warnings from parliament's legal department that a unilateral change in the closure plan would breach the terms of Lithuania's EU accession treaty, approved in a binding referendum in October 2003.

European Commission spokesman Ferran Tarradellas Espuny repeated the EU executive's line that Lithuania had agreed to close the plant during its accession negotiations and could only change this with the agreement of other EU states.

"Of course it is Lithuania's right to have a referendum but it is not going to change anything," Tarradellas Espuny said.

"MISLEADING"

Rita Grumadaite, spokeswoman for Lithuanian President Valdas Adamkus, said Adamkus felt a referendum would be "misleading" for Lithuanians since it would not be binding, although the president had no powers to veto parliament's decision.

But she added that Adamkus thought it could be good to extend Ignalina's lifespan until a new plant is built -- "given the worrying energy situation" -- if it were possible without violating the accession agreement.

Lithuania has said it wants to build a nuclear plant of 3,200-3,400 megawatt capacity by 2015 in cooperation with neighbouring Latvia, Estonia and Poland, although many experts view that date as optimistic.

Lithuanian Prime Minister Gediminas Kirkilas has said he would like to keep Ignalina open at least until 2012.

"We should not remain silent and wait until the others take our electricity market share," said Vesaite, of the Kirkilas-led Social Democrat Party.

Asked about possible EU sanctions if Ignalina is not closed, Vesaite said: "I am not concerned about that because we are going to suffer similar losses due to increased energy prices."

A referendum would only be valid if at least 50 percent of voters turn out, and more than half of them approve the measure.

Given turnout of 46.8 percent for the last parliamentary polls in 2004, that could mean a vote is hard to win even if most Lithuanians favour keeping Ignalina running, political analyst Algis Krupavicius said. He said parliament will be free to decide whatever the outcome of the referendum.

Ignalina's remaining 1,300 megawatt reactor meets some 70 percent of the Baltic state's electricity needs at cheap rates. Without it, Lithuania, whose only power interconnection to the EU is via Finland, will have to import more Russian gas. (Additional reporting by Mark John in Brussels; Editing by Catherine Evans)