The energy giant Npower has been fined £1.8m after a Sunday Times investigation revealed that sales staff were routinely lying to customers in an effort to persuade them to sign up with the company.

Ofgem, the industry regulator, announced the penalty last week – the biggest fine incurred by the firm – claiming it had breached its licence by failing to protect customers.

Although Npower claims it has cleaned up its act, potential customers have continued to contact this newspaper with allegations of sharp practice.

Brian Pullee, 73, a hotel owner from Snowdonia, north Wales, said he was called in October by a sales rep who claimed that switching his gas and electricity provider would save him money. Pullee said he would consider the offer but a week later received a letter saying his supplier had already been changed. “I knew I hadn’t signed a contract or agreed to switch on the telephone,” said Pullee.

Three weeks ago a man claiming to be an Npower employee from the Midlands told The Sunday Times that although it was now more difficult for staff to dupe customers into signing contracts, some still exaggerate the potential savings.

Beverley Harrington, a spokeswoman for Npower, said: “We are very concerned to hear your [latest] allegation regarding mis-selling. There is no place for any inappropriate behaviour in our organisation.”

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