EDF Energy said it lost almost 300,000 customer accounts last year as the business faced “extremely challenging market conditions”.

The French-owned gas and electricity supplier said its underlying operating profit slipped 15% to £664 million in 2015, which it attributes to ongoing investment and reflects the loss of 291,000 accounts over the last 12 months as customers switched for cheaper deals.

It added that mild winter weather hit energy demand in the fourth quarter, with the UK seeing the warmest December for at least 100 years.

The figures come as the firm said it will extend the life of four of its eight nuclear power plants in the UK for up to seven years, safeguarding 2,000 jobs and helping with tight national energy supplies.

The firm said it invests £600 million a year in its UK nuclear plants, adding that its investment “is paying off”.

Last week EDF Energy and British Gas owner Centrica became the last of the Big Six providers to lower prices, announcing moves to cut gas bills next month.

EDF Energy is to bring down its standard gas tariff by 5%, making bills cheaper for around 900,000 customers who will on average save around £31 a year.

The reductions follow announcements by rivals npower, SSE, E.ON and Scottish Power in recent weeks.

But the industry has been criticised for being slow to pass on falls in wholesale gas prices, with consumer groups and regulators hitting out at the size and timing of cuts.

Most of the recent reductions will not take effect until early spring, with EDF’s cuts coming on March 24.

The Competition and Markets Authority has been carrying out a wide-ranging investigation into the UK’s energy suppliers since last summer.

Overall, parent firm EDF Group said its net profit last year plunged 68% to 1.2 billion euro (£929 million) due to the impairment of a range of assets based mainly in the UK, Poland and Belgium.

EDF Energy has 5.6 million customer accounts in the UK.