Energy companies have been called on by Ofgem, the regulator, to explain why their retail prices have not fallen further, given the steep drop in wholesale gas and electricity prices over the past year.

Ofgem on Thursday published a letter from Alistair Buchanan, its chief executive, to the heads of the ”big six” energy suppliers, including British Gas, Eon and EDF, warning them that ”in a strong competitive market, we would expect prices to respond to such falls [in wholesale prices]”.