The European Commission has urged countries across the bloc to cut their gas use by 15% from August to March amid fears Russia could halt supplies.
It says the target is voluntary but will become legally binding if Moscow turns off the taps this summer.
The key Nord Stream 1 pipeline from Russia to Germany has been offline for maintenance for 10 days and is due to be turned back on this Thursday.
But there are concerns Moscow will not follow through on its promise.
Adding to the uncertainty, Vladimir Putin said on Wednesday that it was not clear whether or in what condition a turbine from the pipeline would be returned in after repairs in Canada.
The Russian president said there was a risk the equipment would have to be switched off at “some point” and Nord Stream 1 would be shut down.
Russia supplied Europe with 40% of its natural gas last year, with Germany the continent’s largest importer in 2020, followed by Italy.
European Commission President Ursula von der Leyen said a Europe-wide cut-off was now a “likely scenario”.
“Russia is blackmailing us. Russia is using energy as a weapon,” she said. “Therefore, in any event, whether it’s a partial, major cut-off of Russian gas or a total cut-off of Russian gas, Europe needs to be ready.”
Since Russia invaded Ukraine it has cut supplies to a number of countries which have rejected its demand for payment for gas in roubles, including Poland, Bulgaria, the Netherlands, Denmark and Finland.
It has also been accused of reducing supplies to make it more difficult for EU countries to build up reserves ahead of winter.
The UK gets less than 5% of its gas from Russia, but its gas prices are affected by fluctuations in global markets.
The Commission said a full cut-off during winter could have a major impact on EU economies, reducing growth by up to 1.5%.
The International Monetary Fund last week warned it could plunge countries into recession, heightening an energy crisis that has sent consumer bills soaring.
European nations have been trying to build up gas storage ahead of winter in case Moscow further restricts supplies.
The proposed 15% reduction is compared with average consumption in the same period from 2016 to 2021.