British Gas is to take on the 176,000 customers of Together Energy, which ceased trading last week.
A British Gas statement said: "In line with the regulatory framework, it has been agreed with Ofgem that relevant costs, including the costs of buying energy incurred by British Gas that it is not able to recover as a result of taking on the customers, will be recoverable through the established industry levy in a timely manner.
"In the coming days, we will be in touch with Together Energy customers about the process of moving over."
Centrica chief executive Chris O'Shea added: "We welcome customers of Together Energy to British Gas, and we’ll ensure the switchover is as smooth as possible.
"The actions we have taken over the past couple of years mean Centrica is in a robust financial position, and as a responsible energy supplier built on a sustainable model we are well hedged for the winter and beyond.
"We have now stepped in to help around 700,000 customers who have found themselves without a supplier over the past year, and we’ll continue to engage with the government and Ofgem to find ways to help stabilise and support the sector."
In November, the business said it was looking to secure "long-term funding for growth" as it aimed to return to profitability, but this was never found and it became the latest supplier to cease trading.
Along with subsidiary Bristol Energy, the company supplies about 176,000 domestic customers and one non-domestic customer.
It is understood to have about 300 staff.
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