A pensioner lived for days with no heating or hot water after being forcibly switched to a prepayment meter for his energy without his knowledge.
The elderly man, who thought he was experiencing a power cut when his energy was cut off, is one of thousands of people to be moved to a prepayment meter by their energy supplier without their consent, on even awareness.
The story was shared by energy regulator Ofgem as an example of the harm inflicted by energy suppliers.
Head of Ofgem Jonathan Brearley said: "During our review of how supplies treat some of the customers we heard directly from an elderly gentleman who thought he'd had a power cut because he not been told that he'd been switched over to a prepayment meter.
"This meant he spent days without light or heat, not knowing what was happening.
"At any time, but particularly in this cold weather, this is obviously unacceptable."
The UK's cost of living crisis has seen utility bills rocket in recent months.
As customers fall behind on their bills, energy companies are forcibly moving users onto prepayment meters.
Prepayment meters work on a pay-as-you-go basis, meaning households have their energy shut off if they are not able to top up.
Brearley said he was concerned about the “sharp growth in households struggling to pay their bills being switched over to prepayment meters, sometimes without their even knowing about it, leaving them without heating”.
It follows Business Secretary Grant Shapps’ demand over the weekend that energy suppliers stop forcing financially-stretched households to switch to prepayment meters.
He also vowed to “name and shame” the worst offenders.
Brearley said a review by the regulator last year of firms’ processes for dealing with the most vulnerable customers resulted in legal orders to improve.
He said Ofgem would be undertaking a more detailed assessment to check if the plans had actually led to improvements.
He said: "It is simply not acceptable that vulnerable customers are left in the dark and cold in winter.
“This review will focus specifically on self-disconnections, remote switching and forced installations, and the checks and balances companies have around any decision to put a customer on a pre-payment meter.
“If we find that they have not taken due care in this process, we will take further legal action against them.”
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On Sunday, Shapps said energy firms should first make greater efforts to help those struggling to pay their bills, such as offering credit or debt advice.
Of the hundreds of thousands who have been switched to prepayment plans in the past year, some have found their smart meters switched to prepayment mode remotely while others have been confronted at their door by teams sent by energy companies – armed with magistrates’ court warrants – to physically make the change.
Campaigners say they often then go without power as they cannot afford to keep the meter topped up – something that is referred to as “self-disconnection”.
Tom Marsland, from disability equality charity Scope, said: “For over a year we have been regularly hearing from disabled people who have been forcibly moved on to prepayment meters because they could not afford the rising costs of energy. But prepayment meters are not the answer for disabled people who are freezing in their homes, or parents who are going hungry to feed their children.
“The government must now ban energy companies from forcibly putting customers on to pre-payment meters, which is cutting off lifesaving heating and electricity for disabled customers."