Energy companies are failing to reveal details of their cheapest deals when people phone up to switch suppliers, consumer group Which? has claimed.
During an undercover investigation, the watchdog called the six major energy suppliers 12 times in one week, with salespeople failing to mention the lowest cost tariff in a third of cases.
The probe found that staff at Southern Electric - part of Scottish and Southern Energy (SSE) - only mentioned its cheapest charges in three of the 12 calls.
Meanwhile, seven of the 12 EDF salespeople recommended its more expensive fixed-rate deals instead of its cheaper web-based option.
Across the big six - British Gas, EDF, SSE, npower, E.ON, and Scottish Power - one third of the salespeople did not mention relevant exit fees.
With energy price increases of up to £190 a year now taking effect, the average annual household energy bill is now £1,300, Which? revealed.
It added that these factors mean more people than ever are looking to switch, with a quarter of them relying on the telephone to do so.
Richard Lloyd, Which? executive director described sales staff failing to give details of the cheapest available deal as "unacceptable".
"Giving the right advice to customers about switching matters more than ever when so many people are struggling with escalating fuel bills and colder weather is starting to bite," he continued. "Our advice to customers is that if you are going to switch, make sure you insist on being told the cheapest possible deal.
"Check for exit fees and ask about paying by direct debit or managing your account online as this will usually get you a discount," Mr Lloyd added.
The expert also urged consumers to use an independent comparison website or phone line to ensure they secure the best deal.
"Switching is actually much easier to do than you might think and can save you money," he said.
Use the Myfinances.co.uk comparison tables to find a cheaper deal on gas and electricity