Renewable energy supplier Pure Planet is to cease trading, Ofgem has said, affecting 235,000 customers.
Colorado Energy has also gone bust, affecting a further 15,000.
Neil Lawrence, director of retail at regulator Ofgem, said: “Ofgem’s number one priority is to protect customers. We know this is a worrying time for many people and news of a supplier going out of business can be unsettling.
“I want to reassure affected customers that they do not need to worry: under our safety net we’ll make sure your energy supplies continue. If you have credit on your account the funds you have paid in are protected and you will not lose the money that is owed to you.
“Ofgem will choose a new supplier for you and while we are doing this our advice is to wait until we appoint a new supplier and do not switch in the meantime. 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 tariff.
“Any customer concerned about paying their energy bill should contact their supplier to access the range of support that is available.”