Independent supplier First Utility has accused the “Big Six” energy firms of over-charging their customers by £4 billion a year.

Research by the company found that 87% of customers have accounts with the “Big Six” suppliers, with 70% of those clients sitting on the firms’ standard variable tariffs.

First Utility calculated that those customers could save £4bn by switching.

Customers in the East of England and the East Midlands could save the most, it said.

£600m increase

Ed Kamm, Managing Director at First Utility, said: “The ‘Big Six’ have been exploiting customers’ loyalty for far too long and the scale of the problem has been vastly undervalued.

“Customers of the ‘Big Six’ who are on their standard variable tariff are mostly unaware of the savings because their suppliers are clever about how they avoid telling them.

“We’re trying to highlight this today because the increase in overspend since last year is shocking.”

First Utility’s previous survey had highlighted £3.4bn of alleged over-charging.

> See First Utility's over-spending map