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.
More top news
Two passenger trains have derailed while crossing a bridge in India, leaving at least 20 people dead and injuring hundreds more.
Actor Pierce Brosnan was stopped by airport security after trying to board a plane with a knife.
Children are risking their lives by playing "chicken" on motorways during the school summer holidays, police say.