The UK's energy suppliers have been accused of 'ripping off' bill payers who use other methods to pay rather than Direct Debit.
Figures from the government have revealed many homeowners paying through alternative methods such as cash or cheque are typically paying more than £114 a month more than those who used automated payments.
The issue has become such a concern nearly 200 MPs have now signed a House of Commons motion demanding an enquiry by energy regulator Ofgem.
A Conservative backbencher has also hit back at claims from the energy firms the higher cost is to reflect the higher cost in processing payments.
In an interview with BBC Radio's Five Live, Conservative MP, Robert Halfon, said: "Energy companies have been ripping the consumer off - have been fleecing the consumer particularly the poor and pensioners, with their stealth tax for some time."
Mr Halfon has claimed 17 out of 32 energy companies charge more money for cash payments. He added this is unfair on many as at least a million people cannot use Direct Debit as they don't have a bank account.
He said: "What they're saying is that they're imposing a cost on the pensioners and the poorest in order to pursue non-payers."
Prime Minister, David Cameron, is taking Mr Halfon's claims very seriously and has promised to investigate whether homeowners paying cash were in some way subsidising those paying by Direct Debit.
Ofgem has also said it is looking to the different costs already. In particular it has flagged an investigation into Scottish Power to see whether the firm has broken any rules.
An Ofgem spokesman said: "Given public concern over these differences we urge suppliers to also look at how they can provide more reassurance to consumers that they are being set fairly."
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