Energy suppliers have been named and shamed by Ofgem over high prices, with Npower charging families on its standard tariffs £261 a year more than those on its best deals.

About 20 million households are languishing on the 11 biggest suppliers’ standard variable tariffs for dual fuel (both gas and electricity), despite all of the companies offering other deals that are significantly cheaper, according to the Ofgem data, according to The Telegraph.

The regulator hopes the publication of the data will encourage customers to shop around and switch to cheaper deals.

Npower had the highest differential between its standard and its variable prices, charging the 59pc or 1.7m of its customers on its standard tariffs up to £261 a year more than those on its best deals.

Challenger supplier Co-operative Energy had the second highest differential, at £245 a year, followed by First Utility, which charges the 9pc of its customers on standard tariffs £157 a year more than its best prices.

Britain’s two biggest suppliers, British Gas and SSE, have some of the highest proportions of customers on their expensive standard tariffs.

Some 74pc of British Gas customers – 6.6m households – are on their standard tariff, missing out on up to £129 a year in savings, while 91pc of SSE customers – 3.9m households – are missing out on savings of up to £98, according to the Ofgem data.

According to the league table the most expensive standard variable tariffs are with two of the smaller suppliers, Extra Energy on £1,130 a year, and Co-op Energy on £1,121 a year.