Ofgem claimed earlier this week that prices paid by consumers had fallen by over £300 million since the beginning of its investigation, however NEA claims that households on prepayment meters still pay an average of £119 more than those using Direct Debit payment.
It adds this figure rises to £257 more than those who pay by online Direct Debit.
Jenny Saunders, chief executive of NEA, told the newspaper there was still a situation "where many low income households are forced to pay higher energy prices than their wealthier neighbours".
"If Ofgem is serious about helping those on low incomes, it should back calls for a mandatory social tariff," she added.
ScottishPower recently announced it was launching a new social tariff to help its most vulnerable customers, claiming the new prices could help people save £211 annually.
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