Extra Energy has become the sixth supplier to cease trading this year.
The Energy Ombudsman said it had opened investigations into 1,160 complaints about Extra Energy over the last 12 months.
Chief Executive Matthew Vickers added: “We are keen to work closely with whichever supplier is chosen to take on Extra Energy’s customers to help ensure a smooth transition for consumers as we did with Octopus Energy following the collapse of Iresa in July.
“If you’re a consumer who has a complaint about Extra Energy open with us, please be advised that we will be in touch in due course with information on next steps.”
Consumer body Which? said while customers will understandably be worried, there is no reason to panic.
Alex Neill, Managing Director of Home Products and Services added: “Your energy supply will not be cut off and will continue as normal. We would encourage affected customers to take a meter reading as soon as they can and when your new energy supplier contacts you, ask for the cheapest deal available.”
Gillian Guy, Chief Executive of Citizens Advice said: “Today’s news and other recent supplier failures again highlight how essential it is that suppliers operating in the market have sustainable business models.”
The news comes as Ofgem has proposed financial and customer service tests for new energy suppliers wishing to enter the market.