Together Energy Retail - and its sub-brand Bristol Energy - have both stopped trading, with 176,000 households affected by the collapse of the firm.
Ofgem has told Brits what to do if they're affected - with now-former customers urged to take a meter reading as soon as possible.
A statement on the Together Energy website reads: "We regret to inform you that the company will cease trading with immediate effect. We want to thank you sincerely for your custom over the past five years."
Customers of Together Energy will now be contacted by a new supplier chosen by regulator Ofgem.
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Ofgem is advising Together Energy customers to take a meter reading and wait until they are contacted by a new energy firm.
Experts say the collapse of another energy company means disruption for customers and rising prices in April.
Citizens Advice head of energy policy Gillian Cooper said: “Together Energy is the 27th energy supplier to have collapsed since August.
"As well as causing considerable disruption and confusion for customers, today's announcement will add to the £2.6billion bill consumers are already facing due to these failures.
“With the price cap expected to go up again in April and the cost of living soaring, many are already facing impossible choices between heating and eating."
