THE SNP has again questioned David Cameron's promise to ensure energy companies charge customers their lowest tariff.

Mike Weir, the party's energy spokesman, accused ministers of being "evasive" over the treatment of those on pre-payment meters.

Asked by Mr Weir about these customers, Conservative MP Gregory Barker said: "They certainly will not be stranded on previous deadweight tariffs."

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Mr Weir accused ministers of "rowing back furiously" on the pledge.

"Pre-payment meters are one of the few examples of paying a penalty for paying up-front," he said. "The UK Government must take action to tackle this matter and should ensure that pre-payment tariffs are reduced to ensure everyone benefits from the proposed change."