Energy companies are giving inaccurate information and dodgy advice to people who ring up to switch providers, consumer watchdog Which? has said.
Despite clearly being asked for the cheapest deal, in nearly a third of calls made to the six major energy suppliers during an undercover investigation the companies failed to offer their cheapest tariff.
Staff also gave questionable advice about potential savings, cashback deals and fixed prices, Which? said.
It found that Southern Electric telesales staff only mentioned its cheapest tariff in three of the 12 calls made to it over the course of one week, while seven of the 12 EDF Energy salespeople recommended its more expensive fixed-rate deals instead of its cheaper online tariff.
Across all the companies, one third of the staff questioned did not mention relevant exit fees, while Scottish Power failed to reveal its £51 exit fees in nine of the 12 calls made to it during the week-long investigation.
British Gas staff offered its cheapest tariff in 10 of the 12 calls but offered wildly varying cashback deals alongside this tariff, ranging from no money to £175.
The watchdog also found that offers varied significantly even within the same region - a caller from one London postcode was offered £125 cashback, while another was offered nothing.
Some salespeople gave conflicting advice on fixed-price tariffs. One advised signing up to Scottish Power's three-year fixed deal because it was so long while another recommended its one-year deal because it was so short.
With price increases of up to £190 a year now taking effect, the average annual household energy bill is now £1,300, Which? said.
The watchdog has found that increasing numbers of people are now looking to switch, with consumers being urged to do so by the Government in recent weeks. And research has found that more than a quarter of customers looking to change rely on telephone advice from energy suppliers to get a better deal.