An angry electricity customer has travelled to London to demand utility companies get their bills right.
Angela Hodgson first thought the massive bill was a joke
Angela Hodgson, 43, of Bolton, is complaining after receiving a bill for £38,292 - even though she has a pre-pay meter and does not pay by bill.
She was handing over an ultimatum from consumer watchdog Energywatch to the industry regulator, Ofgem.
Supplier Npower said there had been a "typing error" and that it had sent a written apology to Mrs Hodgson.
Energywatch said 40,000 complaints about inaccurate bills were investigated last year.
Ms Hodgson said she had been in contact with Npower 15 or 16 times since receiving the bill in November - but was still waiting for the situation to be resolved.
"I thought it was a joke at first but when I rang the firm up, they tried to tell me that I had used the energy," she said.
"The letter was not just an ordinary bill - it was a final demand for the money saying they were sending people to my house to collect the money in a week's time.
"I am lucky because I have a family who I could go to - if I had been on my own, I would have really panicked."
She said she explained to Npower that she was a domestic customer on a pre-pay meter who does not even get electricity bills.
"What really wound me up was it never occurred to them that they may be in the wrong."
When the initial demand arrived, Mrs Hodgson asked her two children and her husband for their advice.
The bill arrived in November 2004
"At first they just didn't believe me - I had to show them the letter. My husband just laughed. They realised straight away it was a mistake.
"I guess it is funny, as long as it isn't you who gets the bill."
Npower said it had contacted Mrs Hodgson in February to explain that the collection notice contained a "typing error", adding it had also sent a written apology.
"We are sorry that Mrs Hodgson has not received our written apology since resolving her billing concerns. We will see this reaches her as soon as possible," the company said in a statement.
Energywatch also investigated several other energy companies for mistakes, including a bill sent seven years late and a customer who was threatened with disconnection for owing £0.00.
Thousands of similar incidents have prompted the watchdog to file a "super-complaint" against the entire energy supply industry.
David Sidebottom, director of Energywatch North West, said: "The complaint aims to end the sloppy billing practices of the energy industry that for too long have caused consumers unacceptable levels of distress and inconvenience."