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, writes The Guardian.

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.

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.

Comparison site uSwitch recently identified 16 tariffs that expired in April, causing bills to go by as much as £414.