Co-operative Energy is increasing gas and electricity prices by 5%.
It will come into effect from 1st April for around 96,000 dual fuel customers, adding £58 to their bills.
It is the energy company’s second price rise in six months – a 3% increase was announced in October 2016.
That hikes the total average dual fuel bill by £90 – up to £1,179 a year.
Price comparison site uSwitch claims Co-operative Energy’s standard tariff will now be £345 more expensive than the cheapest deal on the market.
A spokesperson for Co-op Energy said: “We try to protect customers from price fluctuations wherever possible. In recent months, however, we have absorbed a number of rising costs, including network charges and industry obligations.
“This is no longer sustainable and we have reluctantly taken the decision to pass on some of these costs to customers by increasing the Green Pioneer standard variable tariff by an average of 5%.
“We are in the process of writing to customers to notify them of this change now so that they can switch to a cheaper tariff if they choose.”