Hundreds of thousands of families are sleepwalking into a spring energy bill sting - with up to 418,000 households facing sharp hikes in their gas and electricity costs unless they act fast.
Consumers who locked into a 12-month fixed deal last year are being urged to switch now or risk being automatically bumped onto costly standard tariffs when their plans expire at the end of May.
Comparison site Uswitch.com has sounded the alarm, warning that failing to switch could cost households an extra £259 a year - money that many simply can't afford during a cost-of-living crunch.
The call to action comes as 79 fixed tariffs from major suppliers including British Gas, EDF, OVO, E.ON and Octopus Energy are set to end in the coming weeks.
Unless customers make a move, they'll be rolled over onto their supplier's standard variable tariff - a deal typically priced right up to the energy regulator's price cap, and among the most expensive on the market.
For a typical household, that could mean bills jumping from £1,590 to £1,849 a year overnight - a hit of nearly £260 unless they snap up one of the cheaper fixes still available.
Worryingly, these deals are disappearing fast. The cheapest tariff today - from challenger firm Outfox the Market - stands at £1,590, but analysts warn prices could rise again if wholesale energy costs climb.
The collective cost to consumers could be staggering. If all households affected were to roll onto default rates, the nation could be staring down a £108 million annual bill shock, Uswitch estimates.
But there is a glimmer of hope - in the form of a little-known 49-day rule. Ofgem rules mean households can leave a fixed deal up to 49 days before it ends without paying exit fees, allowing them to grab a better deal before prices potentially rise.
Elise Melville, energy expert at Uswitch, said: "It's rare to see such competitive fixed deals, and the window to act could be short. Anyone on a fixed deal that started last spring should check when it ends - and switch now to avoid being hit with higher bills."
In April and May alone, nearly 472,000 households are expected to change supplier - a sign that many are already racing to dodge the bill hike.
Uswitch is urging anyone unsure of their deal to run a quick comparison and lock in a fixed tariff before it's too late.
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