On Tuesday 30 September eight fixed energy deals will come to an end, bringing in an average annual bill increase of 11% - or £108 - for those consumers who have not switched to a better deal, warns GoCompare.com.
Suppliers with tariffs expiring in just over a week are: EDF Energy, First Utility, Scottish Power, M&S Energy, and iSupplyEnergy.
First Utility customers in north-west England, who are currently on the iSave Fixed v7 September 2014 dual fuel tariff, will see the biggest average bill increase of all, at £237.65, or 21.9%. The biggest increase for a ‘big six' customer will be felt by those living in south-east England who are on Scottish Power's Online Energy Saver 23 tariff, with an average price hike of £164.93.
However some First Utility customers in northern Scotland could actually be in for a price reduction of £6.57 when their fixed deal ends and they roll onto the provider's iSave Everyday standard tariff.
But this doesn't mean that it's a good idea for customers to simply accept First Utility's standard tariff, as they may be able to save even more by shopping around and switching.
Jeremy Cryer at Gocompare.com, said: "With four fixed tariffs from ‘big six' providers among the eight due to end this month, many households could find that their annual energy bills are set to increase if they don't take action and look for a better deal.
The good news is energy regulator Ofgen, has introduced steps to make it easier for people to switch, including a rule that, once you receive notification from your supplier of your fixed tariff coming to an end - usually 42 to 49 days before it's due to expire - you can switch without any exit fees being applied.
"And that's exactly when you should start shopping around, as it can take as much as four to six weeks to transfer to your new supplier," adds Jeremy.