The energy market is still failing the majority of customers who are missing out on savings of up to £400 a year, a watchdog has warned.
Recently announced price cuts by the 'Big Six' suppliers, which will all have come into effect later this month, will save customers on standard tariffs just £30 a year.
But Which? said the same customers could save up to £400 if they were to switch to the cheapest deal on the market, or £260 if they were on a gas-only tariff and switched to the lowest available deal.
The consumer group has urged the Competition and Markets Authority (CMA), which is due to conclude its investigation into the energy market, to make switching between providers and tariffs easier and impose penalties on those suppliers who do not protect the most vulnerable.
As the remaining price cuts from the Big Six come into effect this month, Which? research reveals that the energy market is failing as the majority of customers are still missing out on savings of up to £400 a year.
Government estimates suggest that just 12% of customers currently switch their gas provider.
Which's own figures suggest 71% of gas customers are on standard tariffs and 88% of households remain with the Big Six.
Its latest survey found 63% of people are worried about energy prices and just a third (34%) trust providers.
Which? announced it has submitted a dossier to the CMA with comments from more than 30,000 customers detailing dissatisfaction with energy companies and the market.
Which? executive director Richard Lloyd said: "Millions of people are still paying way over the odds for their energy, and levels of switching, while increasing, are still woefully low.
"We would urge all consumers, especially those on a standard tariff with the Big Six, to switch to a cheaper deal today.
"The stakes are high for the outcome of the energy market investigation and consumers will be expecting action to protect the most vulnerable and bring about fairer energy prices."
Energy and Climate Change Secretary Amber Rudd said: "I have met the bosses of the Big Six and I have challenged them to bring down bills and pass on savings to consumers. All six have now dropped their prices, which is a step in the right direction, and I will continue to champion the families and businesses who pay the bills and work hard to ensure they are kept as low as possible.
"I am on a mission to make sure everyone gets a fair deal on their energy. You can play your part too. Thanks to Government action there are more energy suppliers to choose from than ever before. It's not just about the Big Six anymore, there are new kids on the block and it's about the big 37 and counting.
"More competition means better deals for everyone."