Company bosses had suspended 17 members of sales staff amid claims they had misled customers.

An undercover reporter for The Sunday Times claimed to have found npower salesmen making customers sign forms without telling them they were contracts, lying about standing charges and taking advantage of people who spoke little English.

Several potential customers were falsely told they would save money by switching to the firm, the newspaper said.

Industry regulator Ofgem launched an investigation into the firm on April 22.

An npower spokesman said: "The senior management at npower are treating this extremely seriously and have instigated additional activity to complement our existing sales processes.

"We already operate within a robust framework to ensure we comply with strict regulatory obligations and codes. Our new checks include introducing mystery shoppers and contacting 2,000 new customers by phone for further feedback on our door to door sales teams."

If Ofgem finds that npower was involved in mis-selling, the firm could be fined millions of pounds.

The last company to be penalised for mis-selling was London Electricity, which was forced to pay a £2 million penalty.

The probe is a double blow for npower, with the most recent figures from watchdog Energywatch showing the firm has sparked more complaints that competitors including EDF, E.ON Energy and British Gas.