Russian energy company Gazprom said it has "stopped supplying un-paid-for gas to Ukraine" this morning, according to reports in Russian media.
Gazprom said Ukraine has failed to pay for its gas supplies.
A Gazprom spokesperson told ITV News the company will now require all payments to be made in advance.
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Figures suggest that more than 11 million homes, or 42%, are in credit to their energy company.
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