Shares in Bord Gáis Energy's parent Centrica slumped to a 16-year low today.
This comes after Centrica warned that a UK price cap on energy bills, a fall in nuclear output and lower volumes at its oil and gas division would hit its 2019 results.
Centrica said a regulator-imposed cap on standard energy prices in the UK would lead to a £300m hit to profits in 2019, including a one-off impact of about £70m in the first quarter of 2019.
Late last year Centrica, whose British Gas unit is Britain's largest energy supplier, said it would seek a judicial review of the way regulator Ofgem calculated part of the price cap.
The cap had initially been set around 6% lower than British Gas's standard variable tariff.
Ofgem this month said the cap will be raised by 10% from April 1 and British Gas has already said it will increase its prices by the same amount.
"We believe the cap is the wrong intervention but it is here and we will live with it," Centrica's chief executive Iain Conn told journalists.
British Gas shed 742,000 customer accounts in 2018 as the company came under pressure from smaller, nimbler rivals, often able to offer cheaper deals.
Centrica posted a 12% rise in 2018 operating profit to £1.39 billion, bolstered by higher commodity prices, and maintained its full-year dividend at 12 pence.
But traders focused on the weak outlook and concerns the dividend could be under threat in future.
In exploration and production, volumes at Centrica's Spirit Energy are expected to remain in the lower half of its 45 to 55 million barrels of oil equivalent range for 2019.
Meanwhile, output from the company's 20% stake in Britain's nuclear plants was still affected by current outages, Centrica said.
The company is targeting sales of £500m from the sale of non-core assets in 2019 and said it would sell Clockwork, its North American home services business, for $300m.
Conn said plans to sell its UK nuclear stake are ongoing and the company expects to provide an update on this mid-year.
It also plans to make efficiency savings of around £250m, include making between 1,500 and 2,000 job losses on a like-for-like basis in 2019, which Conn said were part of the 4,000 cuts to 2020 announced last year.
Bord Gáis Energy's revenues rise but profits dip
Bord Gáis Energy saw revenues rise 10% to £907m last year as its customer numbers increased.
However a planned, major maintenance outage at its Whitegate plant in Co Cork pushed adjusted operating profits 6% lower to £44m.
Bord Gáis Energy said this the first major overhaul at the plant since it was commissioned in 2010.
Whitegate came back online in May and the company said it was operating at improved efficiency levels in the second half of 2018 compared to the previous year.
The company said its customer numbers increased by 12,000 last year to 691,000 with growth in both consumer and business accounts.
