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."
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