From Thursday 31 July, households could see an 8% upswing in their energy bills after being automatically shifted onto a more costly standard tariff, consumers have been warned.
Popular tariffs from British Gas, Scottish Power and Sainsbury's Energy, as well as iSupplyEnergy's electricity-only tariff, are all due to expire on that date.
The average bill increase for those who fail to switch to a better deal is £85.80, but ranges from £33.22 to £155.77 a year, according to research from price comparison site GoCompare.
Customers who could see the biggest price rise are those on Scottish Power's Online Energy Saver 22 tariff, living in the South East. However, should they switch to Extra Energy's Fixed Price September 2015 v11 dual fuel tariff instead of forking out £1,208 on Scottish Power's standard tariff, they could save £218.
Early exit fee
Jeremy Cryer, energy spokesperson at Gocompare.com, explained: "When your fixed energy tariff is coming to an end you will be notified by your supplier, and usually around 42 to 49 days before it's due to expire. As soon as you have received this notice you can switch without fear of being charged an early exit fee, so this is when you should shop around to look for a better energy deal.
"Bear in mind that it can still take around four to six weeks to switch suppliers, so it's important that you start to compare the different tariffs available to you sooner rather than later. That way, if you decide to switch you won't be waiting too long before you'll be paying for energy at your new tariff's rates."
For anyone looking to switch providers, GoCompare said the cheapest tariff currently available is Co-operative Energy's Fair & Square September 2015, which costs £989 a year on average.