Scottish Power is to raise its standard variable gas and electricity prices from 8 October.
It said dual fuel prices would increase by an average of 3.7%.
The company said the rise was due to wholesale energy costs having risen by more than 20% since April.
It is the second price increase announced by Scottish Power this year. In April, it said it would raise its gas and electricity prices by 5.5% for a third of its customers from 1 June.
On Friday, Neil Clitheroe, chief executive of Scottish Power’s retail division, said the increase in standard variable prices reflected “rising wholesale energy costs and compulsory non-energy costs”.
The company said the majority of its customers would not be affected as more than two-thirds were on fixed-price products or other tariffs not affected by the price change.
Those affected “will be contacted and offered the opportunity to move to a fixed-price tariff”, it said.
The UK government said it was “disappointing that Scottish Power has announced another price rise in its default tariff at a time customers are already paying more than they need to”.
A spokeswoman for the Department for Business, Energy and Industrial Strategy said: “This is why the government is introducing a new price cap by this winter to guarantee that consumers are protected from poor value tariffs and further bring down the £1.4bn a year that customers have been overpaying.”
Earlier this month, British Gas announced its second price rise this year, with a 3.8% increase in its standard variable tariff (SVT) due to come into effect from 1 October.
Parent company Centrica said the move would affect 3.5 million customers.
E.On, SSE, Npower, EDF and Bulb have also recently hiked energy prices, blaming wholesale energy costs for the increases.
Ofgem has also announced it is raising its pre-payment meter cap – which protects vulnerable customers – from October.