Analysts claim Co-op move to raise bills in October by up to £70 is due to rise in gas import costs driven by slump in pound after EU referendum

Energy experts are warning that household energy prices could be about to rise for the first time in two years, driven partly by higher import costs following the Brexit vote.

The Co-op started the ball rolling when it told some of its 500,000 energy customers that, from 1 October, it would be raising bills by between 3% and 6% – the latter equating to a potential rise of almost £70.

The price of wholesale gas has steadily risen over the last three months but Britain is facing a double hit because gas imports from the continent are about 10% higher still, due to a fall in the value of sterling against the euro.

The Co-op is raising the average bill for dual-fuel customers on a standard plan from £1,152 to £1,184 a year. People with pre-payment meters could find their bills rising from £1,115 to £1,184, according to the price comparison site uSwitch.

“This is a worrying warning bell that the wholesale price honeymoon may be drawing to a close. Wholesale prices are now climbing at the fastest rate in years, driven by upward pressure on the cost of energy imports from the falling value of sterling following the EU referendum, future supply concerns and higher transmission costs,” said Claire Osborne, energy expert at uSwitch. “Unfortunately, it’s the smaller suppliers who are less able to cope as they cannot buy their energy as far ahead as the big six [companies]. The danger is that other small suppliers could now follow suit and raise their prices – just in time for winter.”

Osborne claimed the Co-op service was now more expensive than British Gas, SSE or any other of the big six suppliers. She urged consumers “to fight back” by transferring their business to cheaper firms. But there could be still cheaper deals available from a raft of new independent companies.

Cornwall Energy, an independent energy consultant, confirmed that changes in the exchange rate since Brexit had made power more expensive in Britain. Gas imported from other European countries was used in the home directly and for burning in power stations to produce electricity, it pointed out.

Co-op Energy was unable to immediately comment on its price rises. But critics pointed out that the energy trading group of Midcounties Co-op had also been put at the top of a complaints league by the energy ombudsman last November.

SSE attracted the least complaints, while uSwitch said it now believed that British Gas offered one of the cheapest standard rate tariffs, at £1,102 a year. None of the big six firms have increased their prices in the last two years but they have also being losing market share.

Figures from the industry lobby group Energy UK, indicate that 1.3 million customers moved from a large supplier to a small one during the past 12 months, partly persuaded by bad publicity.

The Competition and Markets Authority at one stage concluded that customers using standard tariffs were wasting collectively as much as £1.7bn a year.

It is still possible to find rates from an independent supplier, such as Avro, of £770 a year, but experts say that the smaller firms are less able to withstand rises in wholesale costs as they do not have the cash to hedge their investments.

A reduction in the market share of the big six, from over 99% to less than 87%, over the last seven years, has also forced those companies to cut prices and improve customer service.