Co-op Energy has “reluctantly” decided to increase its prices by 5.2 per cent for gas and electricity from next month.
The company said it is not “sustainable” to absorb increases in wholesale energy prices as it has tried to do previously.
The changes, which come into effect on 20 August, will impact 65,000 customers on its green pioneer standard tariff.
Customers on a fixed price tariff, prepayment tariff or the safeguard tariff will not see any change.
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 from 20 August.”
Fellow challenger supplier First Utility will increase its prices by an average 5.9 per cent for dual fuel customers on its standard variable tariff from 23 July, following its announcement last month.
Bulb also plans to increase its prices by an average 5.1 per cent from 12 August. All of the big six energy suppliers have announced price hikes this year.
According to Rik Smith, an energy expert at uSwitch.com, this latest rise is the 25th to have happened so far in 2018.
Smith said: “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.
“Despite the warm weather it has been a tough year for energy customers who have seen their bills skyrocket by an average of £58 (5.6 per cent).
“But in the face of this relentless barrage of price rises taking standard tariffs to nearly £400 above the cheapest deals on the market, bill payers have been fighting back in record breaking numbers.
“According to the latest data, 2.7 million customers have switched so far this year, with record numbers of people going to small suppliers.”