FORMER customers of energy firm Npower have received demands for bills months after switching to another provider, a report has revealed.
A survey from moneysavingexpert.com says it had seen scores of complaints, with existing Npower customers also receiving late bills.
One received a bill for more than £1,000, despite leaving Npower last June, with another receiving a £350 demand 16 months after switching.
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Martin Lewis, moneysavingexpert.com's creator, said: "Npower is punishing people for its own mistakes.
"Once people have closed an account that should be it.
"It must do the decent thing and wipe or at least reduce bills to a manageable amount."
Npower said anyone worried about paying a bill should get in contact.
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."