British Gas customers could see a £100 drop in gas and electricity bills this year – thanks to the warmer weather.

Its parent company Centrica said the mild weather meant Britons kept their heating off for longer, with the average residential gas usage for the first 10 months seen to be 21% lower compared to last year while average electricity consumption fell 7%.

The energy giant has therefore reduced its earnings forecasts on the back of the mild weather and the extended outages of two nuclear power plants it partly owns.

The Heysham 1 and Hartlepool nuclear stations, co-owned with EDF Energy, had to be taken offline for inspections after cracks were found in a boiler. Although expected to return to service soon, they may have to work at reduced power.

“Full-year adjusted earnings per share in 2014 are now expected to be in the range 19-20p, compared to a range of 21-22p at the time of our interim (half year) results,” Centrica said.

The company also saw a fall in the number of residential energy accounts on supply since June – down 50,000 to 15 million – however it said the figure has recently started to grow.