GB Energy Supply has ceased trading, the company announced on its website.
The low-cost energy supplier is believed to have had around 160,000 customers.
Managing director for GB Energy Supply blamed rising energy prices for their demise.
And he asked customers not to phone their call centre, saying the firm would contact those affected in due course.
Ofgem has moved to reassure customers that they will not be cut-off following the announcement.
A statement on GB Energy Supply’s website, said: “It is with regret that we are informing you that GB Energy Supply is no longer trading and therefore you will be unable to access our website at this time.
“Due to swift and significant increases in energy prices over recent months and, as a small supplier our inability to forward buy energy to allow us to access the best possible wholesale prices, means that the position of the business has become untenable and as such we will now be entering a process overseen by Ofgem to move you to a new supplier.
“Customers will be contacted soon by their new supplier with more information on how this process will affect them.”
Rachel Fletcher Ofgem’s Senior Partner for Consumers and Competition said: “In any competitive market companies will fail, this is why we have procedures in place to ensure customers’ energy supplies are always secure.
“If you are a customer of GB Energy Supply there is no need to worry as we will ensure your energy supplies are safe. Ofgem are actively working with the industry to ensure your transition to a new supplier is as smooth as possible. While this process is underway our advice is not to do anything as you can continue to rely on your energy supply as normal.”
“We will work to appoint a new supplier as soon as possible and they will be in touch with you.”