Kozloduy was described as one of the world's most dangerous plants
Bulgaria's President Georgi Purvanov has suggested that a nuclear reactor deemed unsafe by the EU could be restarted to help cover gas shortages.
Bulgaria is one of several countries to have reported falling gas supplies after Russia reduced its exports amid a contractual row with Ukraine.
Mr Purvanov said Bulgaria needed to reactivate the Kozloduy unit as "a more critical situation is hardly possible".
The European Commission said it had not received a formal Bulgarian request.
Ferran Tarradellas, spokesman for the EU energy commissioner, told the BBC that Bulgaria met its commitments to shut down reactors three and four at Kozloduy as part of its accession to the EU, and the EU saw no reason to reverse that process.
Referring to reactor three on Tuesday, Mr Purvanov said "preparations for a restart of the reactor must begin immediately".
Mr Tarradellas stressed that "Bulgaria is facing a problem of gas supply - this is certainly a problem that concerns the EU and we're doing what we can to help Bulgaria receive the gas that it is due from Russia".
The EU has urged Russia and Ukraine to resolve their differences as soon as possible so that gas supplies can be resumed in full.
Feeling the cold
At least two towns in eastern Bulgaria - Varna and Dobrich - were totally without gas on Tuesday, the AP news agency reported.
Firewood sellers in Sofia are preparing for greater demand
Some Bulgarian readers told the BBC News website the effects were beginning to become apparent.
"I can report that the central heating system for Sofia has just stopped working. It's -3C degrees outside at the moment and my apartment is noticeably cooling," said Nev Nikolova in the Bulgarian capital.
Bulgaria made a formal plea for the reopening of the two Kozloduy reactors in March 2007, with the support of regional neighbours, which said they feared economic and political instability otherwise.
The EU rejected the request.
Kozloduy supplied about half of Bulgaria's electricity, before four of its six reactors were closed amid safety concerns. The US department of energy described Kozloduy as "one of the world's most dangerous nuclear installations".