The UK's financial regulator has fined Carphone Warehouse more than £29m for insurance mis-selling.
The Financial Conduct Authority (FCA) found the firm failed to train staff properly to advise customers buying Geek Squad mobile phone insurance
The FCA was tipped off by a whistleblower.
Carphone Warehouse's boss said the firm was "disappointed" it had fallen short in the past but that it was a "very different business today".
Mark Steward, executive director of enforcement and market oversight at the FCA, said: "The Carphone Warehouse and its staff persuaded customers to purchase the Geek Squad product which in some cases had little to no value because the customer already had insurance cover. The high level of cancellations should have been a clear indicator to the management of mis-selling.
"Without whistleblowers coming forward, these practices may never have come to light. In the past few years, whistleblowers have contributed critical intelligence to the enforcement actions we have taken against firms and individuals."
Alex Baldock, Carphone Warehouse chief executive, said: "As the FCA acknowledges, we've made significant improvements since 2015. We're committed to stay on that trajectory, and to make sure all customers enjoy the right technology products and services for them."
The FCA said the company had failed to give its sales consultants the right training to give "suitable advice" to Geek Squad customers.
For example, it found that staff were trained to recommend Geek Squad to customers who already had cover, for example through their home insurance or bank accounts.
Customers who said they might have a similar product or wanted to think about it were advised to purchase Geek Squad and cancel in 14 days.
"This created a risk that customers would purchase insurance that they did not need and would be exposed to the risk of paying for it if they did not cancel in time," the FCA said.
During the period under investigation, between 1 December 2008 and 30 June 2015, Carphone Warehouse sold Geek Squad policies worth more than £444.7m.
The FCA said a high proportion of these policies were subsequently cancelled early. In January 2014, 35% of policies were cancelled within the first three months.
"High cancellation rates are an indicator of a risk of mis-selling which The Carphone Warehouse failed to properly consider," according to the FCA.
Carphone Warehouse did not contest the FCA's findings. As a result it qualified for a 30% discount on the fine, without that it would have had to pay more than £41m.