Analysis by Comparethemarket found switching from a standard variable or default tariff to the most competitive fixed or variable rate tariff can save the average household more than £300 a year.

The cap, which is revised bi-annually and is currently £1,179, is the maximum price suppliers can charge a typical dual fuel customer on a standard variable tariff. The latest limit came into force in October and is £75 lower than the previous cap.

However, the average price for the top 20 cheapest available tariffs on the market is £855, according to Comparethemarket, meaning people could be £324 better off by switching.

The comparison site says 11 million people are stuck on standard variable or default tariffs, and they collectively miss out on £3.5bn in savings by not switching to a competitively priced tariff.

Peter Earl, head of energy at Comparethemarket, said: “Our research shows that the price cap has done little to safeguard those people on a standard or variable rate tariff.

“The more affordable prices available on the market to those who shop around clearly show that the level of the price cap is a rip off.  Too few people are switching suppliers, leaving millions of people paying too much for their energy.”

The next price cap revision is due on Friday and it is expected to fall to its lowest level yet due to a drop in wholesale prices.

But even if it does fall, customers could still save more by switching.

Earl said: “Even if the next price cap drops to the lowest level since it was first introduced, people should not see this as a good value price to pay for energy – rather, it is the absolute ceiling of what they should be paying.

“We would encourage anyone on a standard or variable tariff to review their current provider to see if they can find a better deal elsewhere.”