Npower has become the latest energy firm to announce cuts to residential fuel bills following the recent fall in wholesale gas prices.
The watchdog has told firms they must reduce prices
Six million customers across England, Wales and Scotland should benefit after Npower pledged to cut gas prices for consumers by 16% from the start of May.
Electricity prices will come down by 3% from the same date.
British Gas's decision to cut prices from next month has triggered a wave of similar moves by other firms.
Powergen and Scottish & Southern Energy have already announced price cuts, while other providers are widely expected to follow suit.
Powergen has said new customers will benefit from lower prices, but has yet to announce price reductions for its six million existing customers.
Npower said its action showed its commitment to "offer value" to its customers, adding that it did not rule out reducing bills further.
The wholesale price of gas has been falling steadily since last summer.
Regulator Ofgem has publicly warned gas and electricity suppliers that they will face official sanctions if they fail to pass on wholesale reductions to their customers.