Britain's major gas and electricity supplier Npower is to raise energy bills by an average of 64 pounds (86.72 US dollars) a year for a million customers, representing an average hike of 5.3 percent, the German-owned company announced Friday.
Npower has become the latest energy provider to announce price hikes, which will affect around one million customers.
Citizens Advice chief executive Gillian Guy said: "At a time when many energy customers are struggling with their bills, this price rise will be hard to stomach for the approximately one million customers affected".
"This also means five out of the Big Six have had price rises in 2018".
Npower has stated that the price increase is due to a rise in wholesale costs and Government taxes. It says it is widely acknowledged that these have continued to increase since its last price hike in March 2017.
Managing director of domestic markets Simon Stacey said the decision "wasn't taken lightly".
British Gas is increasing prices by 5.5 percent from May 29, while Scottish Power is raising prices by 5.5 percent on June 1.
It comes as Big Six energy firms face sustained political pressure over the issue of customers paying too much, which has prompted plans by the Government to impose a cap on standard tariffs. We continue to encourage all our customers to look at our range of competitive fixed deals and switch to a tariff that best suits their needs - whether that's our green product or a short, medium deal, or the market's longest fix.
However, customers on a fixed energy tariff, on a prepayment meter or on a "Safeguard tariff" will not see prices rise.
Challenger firm Bulb said that wholesale costs had already increased 13% since it put up prices 2.8% in February.
The easiest way to beat energy price hikes is to use a comparison tool to check if you can switch and save.
Although it's not confirmed prices will rise in August, it said in an email sent to customers: "We promise to give you 60 days" notice if we do increase prices.