British Gas owner Centrica has blamed the new price cap in the UK as it swung into a major loss for the year.
The group's operating loss reached £849m in 2019, down from a profit of £987m in 2018.
The company's adjusted operating profit, which strips out some charges, fell 35 per cent to £901m.
The price cap on bills for customers on default tariffs was introduced at the beginning of 2019. It reduced Centrica's revenue by £300m. Along with other reductions, adjusted revenue fell to £26.8bn, a 2 per cent 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."