Shares (Berlin: DI6.BE - news) in British Gas owner Centrica (Frankfurt: A0DK6K - news) have fallen sharply after it warned 2019 financial performance would be hit by factors including the energy price cap.
The FTSE 100-listed group was down 12% after it also revealed that it had shed 742,000 UK customer accounts last year in a "highly competitive" market.
Centrica said profits at its UK home energy supply division were down by 19% to £466m for 2018, though the overall group's headline measure of adjusted operating profit was up 12% to £1.39bn.
The group had said in November that the cap on default energy tariffs, introduced at the start of January, would have a one-off impact of £70m in the first quarter of 2019.
In its latest statement it said that the impact of the cap, together with a declining performance for its energy exploration and production division and nuclear arm, would see cash flow about £300m below target for the year as a whole.
The company also said it was selling its North American franchisee home services business Clockwork Inc for $300m after a slower than expected recovery for its operations in the region last year.
"We are taking actions to strengthen the company in 2019 and improve underlying performance in 2020, including driving cost efficiency hard and delivering further divestments."
The results come after regulator Ofgem introduced a cap on default energy prices following years of political pressure, which came into force on 1 January and promised to save customers a typical £76 a year.
It had an immediate impact on British Gas, the UK's biggest energy supplier, as the cap was set at a level £68 lower than its standard variable tariff (SVT).
However the regulator said just weeks later that the cap would rise on 1 April by an average £117, blamed on higher wholesale gas and electricity costs.
All of the UK's so-called "big six energy" suppliers including British Gas have now followed suit, lifting their own SVTs to the newly increased cap level.
Centrica reiterated in its latest results that it does not believe the price cap is a "sustainable solution for the market" and was "likely to have unintended consequences for customers and competition".
Analysts pointed to fears that the group's dividend could be cut being behind its sharp share price fall.
George Salmon, equity analyst at Hargreaves Lansdown (Frankfurt: DMB.F - news) , said: "The bad news for Centrica is that the weaker outlook comes from a multitude of factors - the government's price cap, continued outages in the nuclear business and weak offshore production activity.
"This all means the dividend is starting to creak. We wouldn't be surprised if a cut was around the corner."