Russia's Gazprom has declared force majeure on gas supplies to Europe to at least one major customer, according to the letter from Gazprom dated July 14 and seen by Reuters on Monday.
The letter said Gazprom, which has a monopoly on Russian gas exports by pipeline, could not fulfil its supply obligations owing to "extraordinary" circumstances outside its control.
It said the force majeure measure, a clause invoked when a business is hit by something beyond its control, was effective from deliveries starting from June 14.
A trading source said the letter concerned supplies through the Nord Stream 1 pipeline, a major supply route to Germany and beyond.
Gazprom had no immediate comment.
The measure will likely escalate tensions between Russia and the West over the Russian invasion of Ukraine, action Moscow calls a "special military operation".
The European Union, which has imposed sanctions on Moscow, aims to stop using Russian fossil fuels by 2027 but wants to supplies to continue for now as it shifts away from Russian supplies.
Russian gas supplies have dropped via major routes, including via Ukraine and Belarus and through Nord Stream 1 under the Baltic Sea.
Nord Stream 1 is currently undergoing maintenance.