Co-op Energy is raising its gas and electricity prices by 5.2 per cent from next month, a move that will affect up to 128,000 households.
The change, which takes effect from 20 August, will see bills for customers on the standard tariff rise by £61, from £1,158 to £1,218 – a total of £7.8m nationwide.
The bill hike will also impact GB Energy customers, who have been supplied by Co-op Energy since GB ceased trading in 2016.
A spokesperson for Co-op Energy said: “As the largest member-owned energy supplier in the UK, our customers are at the heart of everything we do. That is why we do our best to protect them from price fluctuations wherever possible.
“For that reason, we were the first major energy supplier to automatically move customers onto a new fixed-price default tariff rather than our variable tariffs, and why we have sought to absorb the significant increases in wholesale energy costs this year.
“However, this is not sustainable indefinitely and we have therefore reluctantly taken the decision to pass on some of these costs to customers on our Green Pioneer tariff.”
Rik Smith, uSwitch.com energy expert, said: “The news from Co-operative Energy today is the 25th overall increase announced this year. We have now seen at least one price rise from Britain’s ten biggest energy suppliers in 2018 as well as hikes from numerous smaller providers.”
Mr Smith said that in spite of the recent warm weather, “it has been a tough year for energy customers” who have seen their bills skyrocket by an average of 5.6 per cent, or £58.