British Gas is to take on the 176,000 customers of Together Energy which ceased trading last week.
The supplier, which is co-owned by Warrington Borough Council, includes subsidiary Bristol Energy.
British Gas has said it will ensure the customers continue to receive an uninterrupted supply of gas and electricity.
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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. British Gas will do everything it can to minimise those costs.
"In the coming days, we will be in touch with Together Energy customers about the process of moving over.
"We understand how unsettling a time this may be, but want to reassure our new customers that there is no need to contact us just yet as we are working through getting customer accounts set up.
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"We’ll be in touch as soon as we can to give our new customers more information on the next steps but in the meantime ask that they refer to our dedicated website."
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.
"We’re a brand trusted by millions and our new customers will benefit from the range of advantages that come with being a British Gas customer, including access to exclusive offers on services like boiler cover, Hive products and British Gas Rewards.
"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."
Warrington Borough Council first began its partnership with Together Energy in September 2019, with an £18m investment.
In November 2021 the Scotland-headquartered business said it was looking to secure "long-term funding for growth" as it aims to return to profitability.
Earlier this month, Warrington Council refused to comment on reports the company could be set to collapse.