Three in four households are paying up to £250 a year too much for their energy, according to Labour.
The party says its analysis of official figures shows that David Cameron's pledge to ensure consumers paid the cheapest rates had failed.
More than than two years ago the Prime Minister promised to legislate to force all suppliers to put all customers on their lowest energy tariff.
The Labour analysis show that 75 per cent of households are on their supplier's standard variable tariff, which is, on average, more than £180 a year more expensive than the cheapest. Some customers are paying almost £250 a year more.
Labour reiterated its call for a beefed-up energy regulator and given the powers to force energy suppliers to cut bills.
The party has also pledged that it will freeze prices so that bills cannot rise until 2017 if it wins May's General Election.
Tom Greatrex, Labour's shadow energy minister, said: "More than two years after David Cameron promised to make energy companies put all customers on the lowest tariff, millions of households in Scotland are still being routinely overcharged by hundreds of pounds every single year."
The rates the party analysed were for dual-fuel customers with average consumption and paying through direct debit.
Meanwhile, the Liberal Democrats have outlined plans to insulate up to 10 million homes by 2025.
The party said that the move would help ensure more people benefit from permanently warmer homes and cheaper energy bills.
The party says it would reform the current 'pay as you save' Green Deal scheme to a new 'Green Homes Loan Scheme'.
Lib Dem leader Nick Clegg said that his party had relentlessly pushed the green agenda over the last five years in government "in the face of strong resistance" from the Conservatives.
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