Bulb Energy, which has 1.7m customers - including, it is thought, many thousands of landlords - is to be put into administration.
It is the seventh-largest UK energy company, and easily the biggest to face difficulties following a sharp rise in wholesale gas prices this year.
Bulb will become the first energy company to be placed into "special administration" where it is run by the government - temporarily at least - through the regulator Ofgem. This measure is only used if Ofgem is unable to find another company to take over an energy firm's customers.
Customers have been advised not to take any action and they will be contacted when any steps are needed.
"If you're a Bulb member, please don't worry as your energy supply is secure and all credit balances are protected" a company spokesperson says.
Sarah Coles, analyst at business consultancy Hargreaves Lansdown, says: “Bulb customers have lived with rumours of imminent collapse for weeks. The government has been trying to put contingency plans in place, after it emerged the company was under threat, but it seems these plans have reached the end of the road.
“The level of debt held by the company could mean it is effectively part nationalised for a period, so the taxpayer is getting involved in the currently unrewarding business of energy supply.
“According to Ofgem, in cases like this you will be free to switch to another supplier, and your credit balance will be protected. It’s unclear whether your current tariff will be honoured or whether you will end up being moved onto one linked to the energy price cap.
“It’s clear that with wholesale prices going through the roof, it’s proving nigh-on impossible for companies to make money under the current price cap. It raises the risk that we could see a significant hike in April next year.”
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