Customers could be missing out because one of Britain's biggest energy firms tells them about cheaper deals only if they have opted to receive advertising material.
EDF Energy does not tell customers about cheaper deals on annual bills if they do not opt in to get the advertising material, blaming data protection rules.
In a letter to one customer on a standard tariff this week, EDF said: "As you've chosen not to receive marketing messages from us, we can't tell you about our other options to save you money", the Daily Telegraph said.
Shadow energy secretary Caroline Flint told the newspaper: " Hard-pressed consumers should not have to receive nuisance marketing just to find out if they are being overcharged."
An EDF spokesman said the situation arises because of data protection.
She said: "We let people know what the cheapest tariff is for them via their annual statement, based on their consumption.
"We also do this with letters we send out to let people know their fixed deal is coming to an end. However, if a customer has opted out of 'marketing' Data Protection Act rules mean we cannot highlight the cheapest deals within the letter informing them of the closure.
"We remind them to opt back in if they'd like to hear about our other offers and cheapest tariffs."