The energy regulator Ofgem has written an open letter to the Big Six energy companies asking them to explain to customers why their bills have not fallen despite a drop in wholesale energy prices.
But the firms are saying it is more complicated than it would seem because much more than wholesale pricing goes into making up the cost of our bills.
The government said the companies would do well to listen to the letter or face action - the toughest of which would be a full competition inquiry.
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"You are one of the silence breakers and we all owe you a debt of gratitude."
Maximum temperature 11 C (52 F).
A cut in stamp duty to help first-time buyers is likely to push prices by at least the amount the reduction is supposed to save, MPs said.