More than 230,000 households have been moved to E.on Next after the energy supplier picked up the pieces from three failed rivals in the sector.
Regulator Ofgem said it had appointed E.on Next to take on customers who were once on the books of Enstroga, Igloo Energy and Symbio Energy.
Between them the three companies had 233,000 customers.
They will all be switched over by E.on, and can continue to use their gas and electricity as before.
Ofgem director of retail Neil Lawrence said: “We understand that this news may be unsettling for customers, however they do not need to worry. Their energy supply will continue as normal, and customer credit balances will be honoured.
“Your energy supply will not be interrupted. E.on Next will be in contact with customers over the coming days with further information.
“Once the transfer has been completed, customers can shop around for a better deal if they wish to.”
The three collapsed suppliers are among the nine casualties in the sector since the beginning of September.
It has included some tiny suppliers, such as MoneyPlus Energy and Enstroga, who had just 15,000 customers between them, but also larger players like Avro, which supplied 580,000.
Ofgem has also said several players in the sector have not paid into various funds and could have their licences revoked if they do not cough up.
Around 1.7 million customers have been impacted by the collapses, which have been largely caused by a spike in gas prices, which is squeezing suppliers.
Over the weekend The Telegraph reported that Ovo Energy is potentially interested in making a deal with Bulb, something which could bring together two of the country’s biggest energy suppliers.
Bulb said: “From time to time we explore various opportunities to fund our business plans.”