Ofgem said on Monday that British Gas will take on 15,000 domestic and 8,000 non-domestic customers from collapsed Tomato Energy.
Funds that current and former domestic customers of the supplier have paid into their accounts will be protected, where they are in credit, it said.
For existing Tomato Energy customers, energy supplies will continue as normal after they were switched over to British Gas on Sunday 9 November 2025.
Ofgem said Tomato Energy customers will be contacted over the coming days about the changes. If they wish to switch supplier, they can shop around but are advised to wait until the transfer has been completed.
Rohan Churm, director for financial resilience and controls at Ofgem, said: "Minimising the disruption to customers is our top priority when a supplier exits the market, so I am pleased to confirm British Gas has been appointed to take on customers of Tomato Energy, and will work to move their accounts in the coming days and weeks.
"While I know customers may be concerned, they do not need to worry - any domestic credit balances are protected and there will be no interruption to their supply during this period."
"Tomato Energy customers will be placed onto a competitive tariff, and when the switch is complete they can consider what's right for them and can shop around for other deals. They will also face no exit fees if they choose to switch to another supplier."
British Gas is owned by Centrica.