The 6% dual fuel increase will add £1.32 on average per week to bills from 31 August. This will take standard variable tariff bills to £1,228 a year.

This is the second time EDF has hiked prices, after announcing in April that electricity prices would increase by 2.7% from 7 June, adding £16 to dual fuel bills.

Both times, the supplier cited rising wholesale energy prices behind the unpopular move.

It said wholesale energy prices have increased 18% since the start of the year, and 13% since it last hiked prices in April. The ‘Beast from the East’, reduced gas storage stocks over the winter and global oil markets have all played their part in the increased costs.

EDF Energy managing director of customers, Béatrice Bigois, said: “We know that another price rise will not be welcome, and we had hoped our limited changes announced in April would be enough. However, energy costs have continued to rise significantly and despite our best efforts to absorb some of these by reducing the costs within our control – sadly we can no longer sustain this.

“Customers who wish to avoid this increase will be encouraged to choose one of our fixed price tariffs when we write to them later this month.”

It added that 40% of customers are affected by the changes (1.3 million), while the remaining 60% (1.9 million) comprise those on fixed tariffs, pre-payment tariffs or the safeguard tariff for vulnerable customers so are unaffected.

‘Hitting customers while they’re already down’

Kevin Pratt, consumer affairs expert at MoneySuperMarket, said: “Wholesale costs have been increasing but this is the second price rise from EDF in less than three months and pushes their standard variable tariff right to the top in terms of most expensive – at £1,228 just a fraction cheaper than Npower.

“The previous rise was relatively small compared to the other Big Six suppliers, but it was still a rise and by announcing another one today, it is hitting customers when they’re already down.

“We’re in the height of summer and enjoying a massive heatwave, but this announcement shows that price rises, either from Big Six or emerging suppliers, can happen at any time, and usually when customers least expect it.”

Pratt said customers should take control of their energy bills by switching, helping to save £250 or more. See YourMoney.com’s Energy switching guide for more information.