Customer switching between energy companies has hit a four-and-a-half year high but two-thirds of all households remain on the most expensive tariffs, Ofgem figures show.
The regulator said 5.5 million switches took place this year to September, a 28% increase on the same period last year.
In September alone, around 15% of electricity and gas customers switched suppliers as switching rates reached their highest level since February 2012.
However, 66% of all households remain on expensive standard variable tariffs.
The cheapest deal currently on the market at £822 a year is around £250 less than the average large supplier standard variable tariff of £1,066, Ofgem said.
Ofgem has called on suppliers to reach out to those who have not switched and offer them a better deal, particularly disengaged loyal customers who are still on expensive standard variable tariffs, ahead of the delivering reforms set out by the Competition and Markets Authority.
Ofgem senior partner for consumers and competition, Rachel Fletcher, said: "We welcome the fact that more consumers are switching to take advantage of the cheaper deals or better service on offer.
"But suppliers must do more to reach out to people who have not switched and offer them better deals so that competition drives down prices for everyone, including loyal customers. Otherwise public trust will not be restored in the energy market."
Meanwhile, Extra Energy has come bottom of the latest Citizens Advice complaints table for the third consecutive quarter as customers continued to report billing problems.
The firm has ranked lowest in the table, which measures how well energy suppliers are handling customer complaints, since the start of 2016 although it has improved slightly on the last quarter.
Scottish Power and npower continued to improve their performances, with both of their complaints ratios dropping by around a third on the last quarter.
Newcomer Places for People Energy replaced SSE at the top of the table with a record score of 19 complaints per 100,000 customers.
Citizens Advice chief executive Gillian Guy said: "All energy suppliers should be delivering a first rate service as standard and resolving their customers' problems quickly instead of letting them drag on.
"It's good to see that some energy firms are starting to get to grips with billing issues that have been causing such a headache for their customers. It's crucial that all suppliers act fast to clear up these problems sooner rather than later."