Rebel Energy, which supplies around 80,000 domestic customers, has ceased trading, Ofgem said.
The regulator assured all households supplied by Rebel, as well as its 10,000 non-domestic customers, that they will not experience any disruption to their supply.
Under The Supplier Of Last Resort (SoLR) safety net, customers’ energy supply will continue and funds that domestic customers have paid into their accounts, including existing credit balances, will be protected.
The regulator said all Rebel Energy customers will be contacted by their new supplier, which will be appointed by Ofgem in the coming days.
It advised customers to wait until the new supplier contacted them, in the “following weeks”, before switching to another energy firm.
Customers are also advised to take a meter reading to give to the new supplier in order to ease the switching process and protect any credit.
Once they have been contacted, customers can ask to be put on their new supplier’s cheapest deal or look for an alternative deal from another supplier.
They will not be charged exit fees for switching away from their new supplier.
Ofgem said suppliers must now have capital to cover their risks and ring-fence certain areas of their finances such as customer credit balances under new rules.
However, some companies “will still fail occasionally”, it said, with the SoLR safety net ensuring that affected customers faced as little disruption as possible.
Ofgem’s director general for markets, Tim Jarvis, said: “Rebel Energy customers do not need to worry, and I want to reassure them that they will not see any disruption to their energy supply, and any credit they may have on their accounts remains protected under Ofgem’s rules.
“We are working quickly to appoint new suppliers for all impacted customers.
“We’d advise customers not to try to switch supplier in the meantime, and a new supplier will be in touch in the coming weeks with further information.
“We have worked hard to improve the financial resilience of suppliers in recent years, implementing a series of rules to make sure they can weather unexpected shocks.
“But like any competitive market, some companies will still fail from time to time, and our priority is making sure consumers are protected if that happens.”
Alex Belsham-Harris, head of energy policy at Citizens Advice, said: “The failure of Rebel Energy will worry its 90,000 customers.
“With energy costs and household bills rising, this only adds to an already stressful situation.
“Customers of Rebel Energy will still have gas and electricity, and the credit on domestic accounts will be protected.
“If you’re affected, you don’t need to do anything, Ofgem will appoint your new supplier who will contact you in the coming weeks.
“Don’t try to switch suppliers or cancel direct debits, as this can cause issues, and take a meter reading if you’re able to.
“The Government and Ofgem must ensure customers of Rebel Energy who’ve fallen behind on bills continue to access affordable repayment plans.”
