Large numbers of British housegolds are facing the threat of unexpected hikes in their energy bills this month, despite falling wholesale energy prices, according to The Energy Advice Line.
Eight fixed-rate energy tariffs, including popular deals with British Gas, EDF Energy, Npower, and Sainsbury's Energy, are scheduled to expire at the end of August.
Julian Morgan, managing director of the price comparison, switching, and advice service for energy users said customers who failed to switch beforehand would automatically be bumped to their suppliers' standard rate - generally the most expensive.
"The exact increase will vary from supplier to supplier," Mr Morgan said. "It's estimated that the average increase will be £77 per year - for many customers it will be significantly more and some customers might not be affected at all.
"This is why it's important to check with suppliers using a reliable and reputable service like Energy Advice Line, so you know what will happen to your bills from next month.
"For many people, the financially sensible thing will be to switch suppliers and move to a cheaper, fixed-term deal. But whatever tariff you are on, it pays to check and ensure you're not left worse off come September."
Mr Morgan said there were a number of competitive fixed-term deals available but that it was sensible to speak to an expert as cheap tariffs could sometimes conceal hidden charges.
"Some deals seem attractive at first glance, but it's worth checking things like exit fees," he said. "Some suppliers charge you extra if you want to pull out mid-contract and exit fees of £25 per fuel are not uncommon," Mr Morgan said.
"We understand that heating bills are not a top priority in August but failing to take charge of your energy in the run-up to the cooler months ahead could have a significant impact on your household budget.
"It's quick and can be done on the phone or online. With potential savings of hundreds of pounds to be made, it could be the most cost-effective 5 minutes you've ever spent."
The Energy Advice Line is a consumer champion and an independent price comparison and switching service for householders and small and medium-sized businesses. The service enables consumers to quickly and simply compare electricity and gas prices, and to switch to the best available deal on the market.
The service also offers free advice and a contract management service, including alerts to remind business consumer users when their fixed-term energy contracts are about to end.
For further information, visit energyadviceline.org.uk