Shares in Centrica, the owner of British Gas, have tumbled to a near 15-year low after it revealed it was losing more than 90,000 customers a month and faced a £70m hit from the forthcoming price cap.

The UK’s biggest energy company has also been affected by two nuclear power stations being taken offline and outages at its oil and gas fields.

Centrica shares have fallen 18% since July, and are down 57% since the former BP executive Iain Conn was appointed chief executive in July 2014. Conn insisted cost-cutting targets were on track to make £200m of savings this year.

Amid intense competition, British Gas lost 372,000 customer accounts in the four months to October, a period when it hiked its default tariff for the second time this year.

However, analysts said the figures showed a slight slowdown in the number of households taking their custom elsewhere. In the year to date, the number of customers leaving per month had reduced from 100,000 a month to 90,000, Jefferies bank said.

British Gas is still comfortably the largest energy supplier, with 12.16m customer accounts, dwarfing the other big five. It has reduced its exposure to the government’s default tariff price cap, which takes effect on 1 January, by about a quarter. The number of customers on such default tariffs is 3.1 million, down from 4.3 million at the start of 2018.

Despite that shift, the company admitted it still faced a one-off hit of £70m in the first three months of next year, owing to the way the energy regulator, Ofgem, had set the final level of the cap.

The cap is set at £1,137 for a household with typical consumption, compared with £1,205 for British Gas’s default tariff. Peter Earl, the head of energy at comparethemarket.com, said people were “getting wise” to how poorly priced the company was compared with rivals, and the price cap was not the real problem. “Centrica blaming the price cap is a red herring,” he added.

In the short term, Centrica has been affected by closures at the Hunterston B and Dungeness B power stations, owing to inspections of cracks in graphite bricks at the reactors’ core. The company is trying to sell its 20% stake in the UK’s nuclear plants.

In a trading statement, Centrica said oil and gas output had also fallen 5% for the year because of outages. It was also waiting to hear from government on the “full implications” of the UK’s capacity market being suspended last week, it added.

Payments from the government, designed to ensure winter power supplies, were ruled illegal state aid by the European court of justice. Experts at USB bank believe the company was expecting £14m of payments under the scheme this year, or 2% of expected net income.

The negative reaction from investors appears to have been driven by earnings per share for the year now being expected at 11.5p, down from a previous consensus of 12.8p.