A group of MPs have criticised three energy companies for their treatment of customers on prepayment token meters.
They accuse the firms of back-charging those customers after their prices have gone up.
Some householders are put into debt after the firms increase token meter fees to their new rates, according to the consumer watchdog energywatch.
Scottish Power, npower and Powergen have "back-charged" nearly 750,000 customers between them, the group said.
A motion tabled in Parliament criticising the firms has gathered support from 131 MPs.
The Early Day Motion set by Labour MP Brian Donohue says back-charging can force pre-payment meter customers into "overnight debt" through no fault of their own.
Energywatch campaigns director Adam Scorer said: "Three energy suppliers are creating instant misery for hundreds of thousands of consumers.
"People with these ancient token meters think they are paying upfront for their electricity only to find themselves in debt when the meter is recalibrated."
Responding to the criticism, Powergen said it was in the process of replacing 190,000 prepayment token meters with "key" meters which will quickly change to new prices.
The firm said the £21 million upgrade should be completed by the end of this year.
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