Britain’s biggest electricity providers show remarkable unanimity on what is the main driver of rising energy bills: the increased cost of buying gas and power on the wholesale market.

It was a theme that turned up repeatedly in the statements SSE, Scottish Power, Npower and British Gas issued to explain why they had put up bills by an average of 9.1 per cent before winter.

But outside the big six energy companies, there is far less consensus. Figures from Ofgem, the energy regulator, suggest that wholesale costs only went up 1.7 per cent over the past year.

British Gas is boosting its profits with millions of pounds made by overcharging customers, it was claimed.

The energy provider has allegedly taken £20m from customers with outstanding credit on their accounts and poured it into its annual accounts over the last year.

A whistleblower revealed that profits from credit, which is built up when customers use less energy than they are originally billed for.