Co-operative Energy is to increase the cost of its standard variable tariff by an average of 5%, it has announced.
The move will affect about 96,000 customers on the company's Green Pioneer deal - its only standard tariff - from April 1.
A spokesman 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."
Co-operative said the increase would not affect the former customers of collapsed company GB Energy it took on and who are on the latter's standard variable tariff.
Households have seen a spate of price rises since the turn of the year, hammering already struggling consumers.
Npower, EDF and Scottish Power are among those to have ramped up prices in recent months, with many blaming rising wholesale costs.
Comparison site uSwitch estimated the Co-operative's increase would add £58 a year to bills, coming after a 3% price rise in October 2016, adding a total of £90 to bills in six months.
USwitch energy spokeswoman Claire Osborne said: "Any price rise is disappointing news for consumers, especially when it affects those people already overpaying on poor value standard tariffs.
"However, two price rises within such a short space of time is a real double whammy for loyal customers.
"With one in three working families already struggling to pay their energy bills - and the cost of living set to rise across the board - any price rise will have an impact on the bottom line for families struggling to make ends meet."
Alex Neill, Which? managing director of home and legal services, said: "Co-op Energy's price rise is another blow for customers, who will be outraged that their energy bills are set to go up when the regulator suggested that inflation busting rises are unnecessary.
"Customers sitting on their standard tariff should look to switch to better deals now.
"Millions of hard-pressed energy bill payers are continuing to suffer due to a lack of competition in this market.
"If energy companies fail to properly engage with their customers, then the Government and the regulator must step in."