Image caption A Ukrainian gas installation - there are worries about winter deliveries
Russia's gas supplies to Poland have dropped by 45%, Poland's state gas firm PGNiG says, amid tensions over Ukraine.
The news came just hours after Poland stopped providing gas to Ukraine through "reverse-flow" pipelines.
The Russian gas volumes were 24% lower on Tuesday and 20% lower on Monday, according to PGNiG. That shortfall prompted Poland to halt reverse-flow.
Poland and Ukraine rely heavily on Russian natural gas. Russia is in a pricing dispute over gas with Ukraine.
Some analysts believe Russia, which stopped gas supplies to Ukraine in June over the pricing dispute, is punishing Poland for sending gas to Ukraine.
Ukrainian forces have been fighting pro-Russian rebels in eastern Ukraine since April, after the separatists declared independence in two regions.
Russia has denied arming the rebels and sending soldiers across the border.
On Wednesday Russia's state gas monopoly Gazprom denied Poland's allegation that it had reduced gas supplies.
Hungary and Slovakia pump much more gas to Ukraine than Poland via reverse-flow, but they have not yet reported a significant drop in their supply from Russia.
Earlier this year Gazprom and Russian President Vladimir Putin warned of consequences if EU member states went ahead with reverse-flow deliveries to Ukraine.