British Gas, owned by Centrica, is putting up its domestic gas and electricity prices from 18 August.
Gas bills will rise by an average of 18% and electricity bills by an average of 16%.
The change will affect nine million households with the average dual fuel customer paying an extra £190 a year.
The company blamed the rising wholesale cost of gas, which has gone up by 30% since last winter.
British Gas managing director Phil Bentley said its bills were being driven higher by the fact that the company buys 50% of its gas on the international wholesale market.