Gas and electricity provider Together Energy Retail has stopped trading, affecting 176,000 households.
A statement on the Together website said: "We regret to inform you that the company will cease trading with immediate effect. We want to thank you sincerely for your custom over the past five years."
Customers of Together and its sub-brand Bristol Energy will now be contacted by a new supplier, which will be chosen by regulator Ofgem.
Together also had one business customer.
Ofgem is advising Together customers to take a meter reading and wait until they are contacted by a new energy firm.
Experts say the collapse of another energy company means disruption for customers and rising prices in April.
Citizens Advice head of energy policy Gillian Cooper said: “Together Energy is the 27th energy supplier to have collapsed since August.
"As well as causing considerable disruption and confusion for customers, today's announcement will add to the £2.6billion bill consumers are already facing due to these failures.
“With the price cap expected to go up again in April and the cost of living soaring, many are already facing impossible choices between heating and eating."
Ofgem director of retail Neil Lawrence said: "Ofgem’s number one priority is to protect customers. We know this is a worrying time for many people and news of a supplier going out of business can be unsettling.
“I want to reassure affected customers that they do not need to worry, under our safety net we’ll make sure your energy supplies continue.
"Ofgem will choose a new supplier for you and while we are doing this our advice is to wait until we appoint a new supplier and do not switch in the meantime. You can rely on your energy supply as normal."
Earlier this year The Mirror reported that Together faced collapse as it struggled to find new funding.
At the time the Scottish-based energy firm was trying to find new funding.
All of Together Energy's power comes from renewable sources, and 90 percent of its staff come from the 10 percent of poorest postcodes in Scotland.
The Clydebank-based energy firm is half-owned by Warrington Borough Council.
Many energy firms have been going out of business because an Ofgem price cap stops them passing the rising cost of gas on to consumers.
Energy providers say the price cap means they have to sell energy for far less than it costs them to supply.
Many energy firms have been unable to take the extra strain and have been forced to stop trading.
How the energy price cap works
This cap limits the amount firms can charge the average customer on their default gas and electricity tariffs - usually variable-rate deals.
This cap is currently £1,277 a year for those on default tariffs and £1,309 for those on pre-payment deals.
It is technically a cap on how much energy firms can ask you to pay for rates.
Ofgem is reviewing the cap in April for England, Scotland and Wales - Northern Ireland has a different system.
When that happens it is likely to rise - meaning households will pay even more for energy.
Energy UK chief executive Emma Pinchbeck has warned that "domestic energy prices are going to go up 45% to 50% in the spring".
According to another expert, energy sector specialist Cornwall Insights, bills could rise by 46% as the price cap could rise to £1,865 a year.
