Customers have been told they will continue to receive energy supplies until a new supplier is found.
Their outstanding credit balances would be protected under its safety net.
Ofgem is now in the process of finding a new supplier to take on Iresa’s customers, who will be contacted individually when the process is complete.
Customers have been urged “sit tight”, hold off from switching suppliers, take a meter reading and wait until the replacement company is in touch.
Ofgem’s interim executive director for consumers and markets, Rob Salter-Church, said: “If you are an Iresa customer there is no need to worry as we will make sure your energy supplies are secure and your credit balance is protected.
“Ofgem is working to choose a new supplier as quickly as possible for you.
“Whilst we’re doing this our advice is to ‘sit tight’ and don’t switch.
“Your energy supply will not be affected and will continue as normal – the thing that will change is that your energy will come from a new supplier.
Earlier this year, Ofgem banned the company from taking on new customers, accepting one-off payments and increasing its direct debits.
It had ordered it to extend its call centre hours, respond to customer emails within five working days, clear a backlog of emails, log all customer problems and offer to put all of its vulnerable customers on the priority services register.
Prior to the company ceasing trading, the watchdog had threatened to revoke Iresa’s licence if it did not meet its demands.