Austria has campaigned for years against Slovakia's reactors
Slovakia will restart a reactor at its controversial Bohunice nuclear plant to cope with a potential electricity shortage caused by the gas crisis.
It shut down the unit at the end of 2008 and the decision is likely to cause anger in neighbouring Austria.
The plant is a complex of five Soviet-type reactors near the city of Trnava, about 50km north-east of Bratislava.
Three older reactors have been decommissioned under the terms of Slovakia's EU accession agreement.
The latest was decommissioned just two weeks ago.
'Close to blackout'
But Slovakia's Prime Minister Robert Fico announced after a special cabinet meeting on Saturday that his country would now restart the reactor.
The reason, he said, is that Slovakia's gas-fired electricity generating stations have been cut off from Russian natural gas.
Slovak officials say the situation is now so bad that Slovakia is close to a nationwide blackout.
Mr Fico told reporters he was aware Slovakia was violating the terms of its EU membership, but said that was better than risking the collapse of his country's electricity grid.
The unit, he said, should be up and running in less than six days.
The decision is likely to cause anger in neighbouring Austria, which has long campaigned against the ageing Soviet-designed nuclear plants of its former communist neighbours.
In 1977 Bohunice's first reactor suffered a major accident and was shut down two years later.
Czechoslovakia's communist government continued with plans to build four pressurised water reactors at Bohunice, which Slovakia has promised to decommission by 2015.