Former Npower customers have complained about demands for large sums of money arriving months after they have switched energy provider, according to a report.
MoneySavingExpert.com said it had seen complaints on both its own forum and Twitter about households switching away from Npower only to receive bills months later.
Some existing Npower customers had also reported bills arriving late, the website said.
One former customer tweeted the website saying they left Npower last June and only recently received a bill for more than £1,000, while another said they had received a £350 bill 16 months after they left.
MoneySavingExpert said it was calling on Npower to wipe or reduce bills for people who had switched away and received them after six weeks.
But it said Npower had replied saying it would not change its existing policy, where it would only waive charges for energy used more than a year ago and where it was at fault for the customer not paying.
Ofgem rules say suppliers have to make "all reasonable effort" to close an account within six weeks of a customer switching away.
MoneySavingExpert.com creator Martin Lewis said: "Npower is punishing people for its own mistakes.
"Once people have closed an account that should be it. With so many people still stretched, to come back many months later and say 'oops, here's your final bill, you actually owe us a fortune and we want it now' is out of order.
"It must do the decent thing - wipe or at least reduce bills to a manageable amount, set up an easy repayment plan, and ensure it puts no one into fuel poverty."
Npower said anyone worried about paying a bill should get in touch, and it would try to find solutions, which could include setting up a repayment plan.
It said: "We wrote to all our customers towards the end of last year apologising for the impact on them of issues we have had with the implementation of our new billing system.
"We are making good progress in dealing with the root causes of this, but remain totally committed to resolving any problems this has created for our customers."