Big energy firms' prices challenged by Ofgem
11 June 2014 Last updated at 00:17 BST
Energy suppliers risk "undermining public confidence" in the market by failing to respond to falling wholesale prices, the regulator has said.
Ofgem said the "big six" gas and electricity companies should explain to customers why falling wholesale prices have not been passed on to household bills.
Under one measure, wholesale prices are at their lowest level for four years.
John Moylan reports.