The 'big six'- British Gas, EDF, e.on, npower, Southern and Scottish Power – were told to drop their prices following concerns that customers are being ripped off.
Some 5.5 million households face fuel poverty, whereby they spend 10% or more of their income on power.
"Parliament has given us the task of trying to create an effective market where competition is the downward pressure on prices," the watchdog said.
The news was sparked by outrage at EDF's huge surge in profits at a time when families are struggling to pay their bills.
EDF announced that its operating profits rose by 8.5% last year to almost £1.6bn – equivalent to £427 for each of the 3.7m UK households to which the French-owned company supplies gas and electricity.