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Millions of energy customers could save hundreds of pounds a year on their energy bills without even switching supplier, according to the regulator Ofgem.

More than 1.7 million Npower customers could save £261 a year, simply by moving to another Npower tariff.

As many as 6.6 million British Gas customers could save £129 a year, according to Ofgem.

However British Gas says it has changed its prices, so the saving is now £43.

The figures also reveal that SSE has almost all of its customers on the most expensive form of tariff.

91% of SSE customers are on standard variable tariffs, compared to just 9% at First Utility.

The number of people stuck on standard tariffs is controversial, as most fixed-rate deals are cheaper.

In the UK as a whole, 66% of households are on standard tariffs.

Standard Variable Tariffs (SVTs) Supplier % of customers on SVTs Difference between SVT and cheapest deal British Gas 74% £129 (now £43) Co-operative Energy 42% £245 EDF 56% £136 E.On 73% £41 Extra Energy 14% £154 First Utility 9% £157 Npower 59% £261 Ovo 35% £67 Scottish Power 50% £129 SSE 91% £98 Utility Warehouse* 94% £150 *tariffs are SVT for pre-payment meters source: Ofgem/prices as at 28 November 2016

However Npower said that customers were unlikely to be able to save £261, as fixed-price deals were usually limited to a certain number of customers.

Once a certain number of people have signed up, the deal is withdrawn.

'Increased choice'

Last month, the business and energy secretary, Greg Clark, met energy suppliers, amid claims that some of them were making large profits from customers who are on standard variable tariffs.

The industry denied profiteering, but has nevertheless come under pressure to reduce its prices for standard tariffs.

Three of the biggest suppliers have frozen their tariffs through the winter as a result.

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Nevertheless Ofgem published its comparison table to highlight the difference between the cheapest ands most expensive tariffs.

"Millions of people across Britain continue to pay too much for their energy," said Mr Clark.

"The measures announced today are a positive step to help more people benefit from increased choice and competition."

Default tariffs

Citizens Advice said the new table would help customers, but that energy firms could still do more to help.

"We'd like to see energy companies do more off their own back to help customers who are on a low income, like moving them to a cheaper deal," said Gillian Guy, the chief executive of Citizens Advice.

Meanwhile the Competition and Markets Authority (CMA) confirmed that is going ahead with plans to help the 10 million households which have been on standard tariffs for more than three years.

From next year, suppliers will have to tell Ofgem which of its customers have not switched tariffs.

If customers consent, they will be sent details of better deals, prompting them to move.