British Gas owner Centrica blamed the new price cap in the UK as it swung into a major loss for the year.
Operating loss reached £849 million for the full year 2019, down from a profit of £987 million in the 12 months before.
The company’s adjusted operating profit, which strips out some charges, fell 35% to £901 million.
The price cap on energy bills for customers on energy suppliers’ default tariffs was introduced at the beginning of 2019. It reduced Centrica’s revenue by £300 million.
Taken with other reductions adjusted revenue hit £26.8 billion, a 2% reduction.
Iain Conn, Centrica’s chief executive who is set to stand down this year, said: “2019 operating profit and earnings were materially impacted by a challenging environment, most significantly the implementation of the UK default tariff cap and falling natural gas prices.
“Against this backdrop Centrica delivered growth in customer accounts, higher net promoter scores, significant cost efficiencies in excess of our target, and full-year adjusted operating cash flow and net debt within its target ranges.
“As expected, performance during the second half was much improved compared to the first half, demonstrating momentum as we enter 2020.”
He added: “2020 will be another busy year as we complete the re-positioning of the company towards the customer, focused on our strengths of energy supply and its optimisation, and on services and solutions centred around energy, with an emphasis on helping our customers transition to a lower carbon future.”