Toto Energy, which supplies around 134,000 domestic customers, has ceased to trade, Ofgem said.
The regulator assured the firm’s customers that their energy supply would continue, prepayment meters could be topped up as normal and outstanding credit balances would be protected.
Ofgem will choose a new supplier to take on all affected customers, who will be contacted about a new tariff.
The regulator advised Toto customers against switching to another energy supplier until a new one has been appointed and they have been contacted in the coming weeks.
Customers should also take a meter reading ready to give to the new supplier.
Philippa Pickford, Ofgem’s director for future retail markets, said: “Toto Energy customers do not need to worry, as under our safety net we’ll make sure your energy supplies are secure and customers’ credit balances are protected.
“Ofgem will now choose a new supplier for you and whilst we’re doing this our advice is to sit tight and don’t switch.
“You can rely on your energy supply as normal. We will update you when we have chosen a new supplier, who will then get in touch about your new tariff.”
The failure of Toto comes after more than a dozen energy suppliers have gone bust in the past two years.
Ofgem recently announced that gas and electricity suppliers could face independent checks on their customer service and financial strength.
Ofgem set out a raft of new rules designed to drive up customer service standards, lower the risk of suppliers going bust and strengthen the safety net in the event of failure.