The big six energy providers are failing to offer their cheapest deals when asked for them, according to a recent undercover probe from consumer champions Which?.

The investigation has revealed multiple failings from customer service operatives. Which? called each of the six major energy suppliers 12 times in one week and asked for the cheapest deal- in nearly a third of the calls the firms failed to offer their cheapest tariff.

The report says staff gave questionable advice about savings, cashback deals and fixed prices. Which? executive director Richard Lloyd said: “If you call an energy supplier asking for their cheapest deal, that’s exactly what you should get. It’s unacceptable for sales staff to give information that’s plain wrong or confusing.

None of the big six were innocent, although npower and E.ON came out on top, with both providers offering their cheapest tariff in most calls. Southern Electric scored poorly as its sales staff only mentioned its cheapest tariff in three of the 12 calls.

Seven of the 12 EDF Energy salespeople recommended its more expensive fixed-rate deals instead of its cheaper online tariff, despite being expected to tell prospective customers there are cheaper deals online.

Richard Lloyd said: “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.”