Millions of households could see their energy bills rocket by as much as £439 as hundreds of fixed-price deals come to an end in July, August and September, figures suggest.
Customers who do not move to a cheaper deal will be automatically rolled onto default standard variable tariffs (SVTs), which usually provide the worst value.
The average increase will be £269 per affected household, but those supplied by iSupply, Ovo and Green Network Energy will see even greater hikes of £439, £367 and £342 respectively, uSwitch calculated.
Some 276 expiring tariffs from 37 suppliers will add £426m to the energy bills of approximately 1.6 million households if they do not switch, the comparison site warned.
Many of those affected are likely to have chosen their fixed deal last summer, when 13 price rises in the space of three months added an average of £71 to the bills of over nine million energy customers, and will likely be turning their attention to summer holidays as they come to an end.
At least 44 energy companies have already increased the cost of their SVTs by up to 17 per cent this year, following Ofgem's decision to raise the level of the energy price cap in April.
Ofgem is expected to announce that the price cap will decrease by around £80 next month, but this lower rate will not take effect until October 1.
Uswitch warned that even after the cap is reduced, customers on default tariffs could still be overpaying by around £300 a year.
Rik Smith, energy expert at uSwitch.com, said: “Energy bills might not be front of mind just as everyone is about to head off on their summer holidays. But over one-and-a-half million households could be in for a nasty shock when they get home if they don't act now.
“There was more than one price rise a week during 2018, and many of those who switched to escape the onslaught then are now seeing their fixed price plans coming to an end.
“It's time to take action to avoid being rolled onto an expensive standard tariff.”