Energy companies will have to offer customers their cheapest gas and electricity packages within the next fortnight.
MP Chris Huhne, the Energy Minister, said energy suppliers would have to voluntarily begin offering their lowest tariffs - and could face new legislation if they fail to comply.
The MP claimed up to eight million households would save money on gas and electricity bills under the move.
He is hoping to offset the announcement of huge rises in energy prices by forcing suppliers to be more transparent about their tariffs.
Letters explaining what the Government expects from suppliers have been sent out - with a clear threat to enforce new legislation by autumn if they fail to comply.
The move comes just after industry watchdog Ofgem announced a huge investigation into energy charges, which will include forcing suppliers to reduce the number of packages they offer to make choosing the cheapest gas and electricity deals easier for consumers.
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