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Edinburgh-based Our Power, which has 38,000 customers, has gone out of business.
Its website said the interests of customers would be protected and power supplies were "secure".
The regulator Ofgem said it would choose a new supplier to take on the customers "as quickly as possible".
It said repayment meters can be topped up as normal and the outstanding credit balances of domestic customers will be honoured.
Our Power joins long list of small providers that have gone bust recently, including Economy Energy, Spark Energy, Extra Energy, Future Energy, National Gas and Power, Iresa Energy, Gen4U, One Select and Usio Energy.
Ofgem director for future retail markets Philippa Pickford said: "Our message to energy customers with Our Power is there is no need to worry, as under our safety net we will make sure your energy supplies are secure and your credit balance is protected.
"Ofgem will now choose a new supplier for you, ensuring you get the best deal possible.
"Whilst we're doing this, our advice is to 'sit tight' and don't switch. You can rely on your energy supply as normal."
She added: "We will update you when we have chosen a new supplier, who will then get in touch about your new tariff."