If customers fail to take action when fixed deals end in May they will end up on tariffs criticised by Theresa May, says Which?

Thousands of people face annual energy bill rises of hundreds of pounds this month when they are rolled on to default energy tariffs that came under fire from Theresa May this week.

If billpayers fail to take action when a series of fixed tariffs come to an end in May, they face hikes of as much as 55%, or £416 a year, in the case of challenger company First Utility.

Consumer group Which? found a total of 17 fixed deals from 10 suppliers expire this month, including a £396 jump for customers with SSE, £373 for Scottish Power, £358 with EDF and £236 with Npower.

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The smallest increase is the £79 extra that people on another Scottish Power fixed tariff will pay when moved on to the company’s standard variable tariff.

About two-thirds of people – 17 million – are on standard variable tariffs, which the Conservatives promised to cap, although they have been unable to promise bills will not still rise.

The increases this month are not directly connected to May’s threat of a price cap, but are indicative of the gaps between the best and worst deals available. Comparison site uSwitch recently identified 16 tariffs that expired in April, causing bills to go by as much as £414.

Pete Moorey, the head of campaigns at Which?, said: “It shows really for us some of the critical issues that need to be addressed with a price cap. With the cap, we would be concerned about people coming off fixed deals and going on to a standard variable tariff, and thinking ‘that’s now been capped, therefore I sam safe to sit on it’, but still paying a premium.”

Moorey said Which? could not support the cap until the government had met five criteria that the consumer group had set out, including ensuring that short-term price reductions are not offset by increases over the long term.

The group also said the next government should ensure any price cap does not harm competition, customer service or innovation such as the smart meter rollout and time-of-day tariffs.

Which? also urged ministers to ensure a cap is time-limited, in a similar way to a ceiling imposed in April for pre-payment meter households, who are used mostly by more vulnerable customers.

Moorey said that from what he had seen so far of the Tory policy, it seemed prices could go still go up. “If six months on [energy regulator] Ofgem looks at it, and there are shifts in wholesale prices and changes to government policy that are increasing costs, then the regulator could review it and change prices over time.”

Separately, the Begbies Traynor consultancy said it had found “significant” financial distress among UK gas and electricity producers and suppliers in the first three months of the year, partly because warmer-than-expected weather had suppressed energy consumption.