At the end of the month, 35 fixed dual gas and electricity tariffs from some of the country's leading suppliers are due to terminate, 33 of which will see bills rise if customers do not switch in time.
However, many customers will be distracted by the Christmas and New Year season, creating a costly annual bill for those who forget to switch in time.
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The average tariff increase for affected households will be £250.92, a 30 per cent increase on current tariffs. However, the worst-hit customers, on an Extra Energy tariff, face a 42 per cent price hike, which will add an extra £331.59 to their annual energy bills.
"The Christmas period is an incredibly busy time of year so it's understandable that the last thing on people's minds is their energy bills," said Ben Wilson, spokesperson at GoCompare Energy.
"Unfortunately, winter is typically when we use the most energy so taking your eye off the ball for a few weeks can really put a serious dent in your finances.
"Winter is also the time of the year when many of us spend the most money, with lots of us splashing out on food and presents – leaving many suffering from a financial hangover in January."
The only way for customers on the affected tariffs to beat the price hikes is "to shop around for a better deal, either with their existing provider or a new one", GoCompare Energy said.
Wholesale gas and electricity prices have risen in the past few months, soaring in November, which has started to lead to a knock-on increase in energy bills.
Last month, Big Six challenger GB Energy ceased trading due to the rise in wholesale prices.