For Britain’s beleaguered households, being squeezed financially from all sides, rising energy bills have been one of the most unwelcome extra burdens. The utility firms were slow to cut tariffs last year when prices on the wholesale markets fell, but they have been quick to raise them this year. First out of the traps was Npower, owned by the German energy giant RWE, which in early January increased its electricity prices by 12.7% and its gas bills by 17.2%.

It is all the more galling, therefore, when Npower employees have been using the most underhand methods to trick households into switching to it from other suppliers. The firm has regularly topped the list for customer complaints to Energywatch, the consumer watchdog. That, however, does not tell the full story of the extent to which its sales team, driven by the lure of fat commissions, prey on the vulnerable to get them to switch suppliers.

Our undercover reporter, working as a trainee saleswoman, witnessed members of Npower’s sales team tricking unsuspecting customers into signing contracts while insisting that they were not selling anything. The company’s representatives purported to be from “the electricity board” rather than a single supplier.

Customers with poor English were deliberately targeted and given false information about the charges they would face. A 91-year-old man and another with learning difficulties were identified as likely victims. Only when their carers complained did the firm take action. In response to the investigation by The Sunday Times, Npower announced on Friday that it was suspending the 17-strong sales team involved and would be conducting its own internal inquiry. “We’re genuinely shocked,” said its spokesman. “We’ve shut down the sales team concerned while we carry out a full investigation. We genuinely pride ourselves on the professionalism of our sales team.”

That is not good enough. Did Npower’s management not wonder what drove an increase in the firm’s customer base, with 170,000 new households signing up last year? That is despite the fact that many of its prices are higher than its competitors. Has it taken until now before the firm or its German owners have felt it necessary to look at why it generates so many complaints or ranks so low in the customer satisfaction tables?

The essentials of modern life - power and light - are being sold with all the ethics of pyramid selling scams, dodgy timeshare deals or old-style double-glazing cons. Firms such as Npower trade on a respectability that some of its sales force does not deserve and regularly abuses.

High prices are already bringing Britain’s supposedly competitive domestic energy market into disrepute. These underhand sales tactics drive down its reputation further. Npower needs to review its methods and the sort of people it employs. Other utility firms, which are far from blameless, need to do the same. Cowboy selling has no place in Britain’s energy market.