Russian energy firm Gazprom has told Poland and Bulgaria it will stop sending gas to the two countries from Wednesday.
Polish state gas company PGNiG said it had been told all gas deliveries would be halted from 08:00 CET (06:00 GMT).
The Bulgarian Energy Ministry also said it had been informed deliveries would be suspended from Wednesday.
It comes after Russia said “unfriendly” countries must start paying for gas in roubles or it would cut supplies.
Poland has repeatedly refused to pay in this way.
PGNiG relies on Gazprom for the majority of its gas imports and bought 53% of its imports from the Russian company in the first quarter of this year.
It described the suspension as a breach of contract, adding that the company would take steps to reinstate the flow of gas.
Bulgaria’s energy ministry said it had taken steps to find alternative supplies but no restrictions on gas consumption were currently required.
The country relies on Gazprom for more than 90% of its gas supply.
Following the news, Poland’s climate ministry said the country’s energy supplies were secure.
Climate Minister Anna Moskwa said there was no need to draw gas from reserves and gas to customers would not be cut.
PGNiG said its underground gas storage was almost 80% full and, with summer approaching, demand was lower.
Poland also has alternative supply sources, including a liquefied natural gas (LNG) terminal in Swinoujscie.
And on 1 May, a new gas pipeline connection with Lithuania is due to open that will give Poland access to gas from Lithuania’s LNG terminal.
Poland was already planning to stop importing Russian gas by the end of the year, when its long-term supply contract with Gazprom expires.