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."
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."