Fuel-save offers may cost you more

An SSE ad, for example, promised consumers would be up to £100 richer if they switched to its capped tariff. But Which? says assuming prices didn't change, an average user would pay £233 more each year than on the cheapest deal. Even with £100 off, they'd be £133 poorer.

Meanwhile customers lured by an Eon ad of two months' free energy were automatically excluded from the normal discounts of 8% for taking dual fuel and paying by direct debit. This left them with an actual saving of just £3.80 on the average annual bill.

The watchdog monitored and analysed energy company ads for more than a year, concluding seven of them could have been misleading.

While it found most of the claims were, "strictly speaking", correct, they could have made the deals and offers sound more tempting than they actually were.

Eon says it's "sorry this ad was confusing" and has withdrawn the tariff, while SSE says it will take Which?'s findings "on board".

75% on most expensive tariffs

In a separate report, Which? also recorded an increase in public distrust towards energy companies over the last six months, with more than half of consumers (54%) saying they do not trust suppliers.

It adds that 75% of customers remain stuck on the most expensive tariffs, despite energy price hikes being one of consumers' top financial concerns.

The consumer group is calling for the introduction of a single unit price to allow consumers to easily compare tariffs and new deals at a glance, as well as a cut in the time it takes to switch suppliers.

Which? energy expert Sylvia Baron says: "Shop around before you commit and be wary of one-off discounts or money-off vouchers. These can be tempting but, as we've seen, £100 off is not so special an offer when the tariff you're signing up for is more than £100 more expensive than the best deal available."