Half a million households could see a 51% increase in their energy bills this summer – if they simply allow their old fixed-price tariffs to roll over.
The warning comes from uSwitch, the price comparison and switching service.
It says the biggest increases will be for Npower customers, who could see their bills rise £399.
It adds that 50 fixed energy plans are coming to an end this summer, more than ever before.
There are 14 popular fixed-tariff deals from eight suppliers ending this month alone. The average price increase for customers whose deals end in July is £274 per year.
The biggest hikes are for those on fixed price tariffs with Npower, First Utility (£364) and EDF Energy (£360).
Fixed tariffs can protect customers against rising prices, but have cancellation charges if they wish to switch to a different supplier.
Energy regulator Ofgem says two-thirds of customers are on standard variable tariffs, which are typically lower than fixed rate tariffs and do not have cancellation charges.
Claire Osborne, from uSwitch.com said: “Switching tariff is incredibly easy and is definitely worth 10 minutes online, on the phone or on an app.
“You can switch without incurring any exit fees 42 days before the contract end date, and comparing and switching takes just minutes to do.”
Ofgem said that last year saw a surge in customers changing energy suppliers, with switching reaching its highest level since 2010, at about 16% of gas and electricity customers.
Of these switches, nearly half (47%) were to small or medium suppliers.
The uSwitch site is owned by property website Zoopla. It provides a free service to consumers, but charges suppliers a fee when it switches customers to them.