ScottishPower is increasing energy prices in a move that will affect 950,000 households.
The company is hiking its standard variable gas and electricity prices for around a third of its customers from 1 June, with impacted households facing an average increase of 5.5%.
ScottishPower attributed the rise to an increase in wholesale energy costs, alongside costs associated with upgrading meters and delivering electricity from low-carbon sources.
Neil Clitheroe, global retail director at Iberdrola, which owns ScottishPower, said: "We will be contacting all customers affected by the price change to give them the opportunity to move to a fixed price tariff alternative and avoid this increase."
The company said the proportion of its customers on standard variable tariffs will fall throughout 2018 because the company will no longer shift customers onto the standard variable rate when their fixed-term products come to an end.
Victoria Arrington, a spokeswoman for Energyhelpline, said ScottishPower's price rise was the most substantial announced so far from the energy sector.
Both British Gas and EDF unveiled price hikes last week, raising their standard variable rates by 5.5% and 1.4% respectively.
The series of announcements means a total 6.35 million households have been saddled with price rises in the new financial year.
"The energy price rise wave is full on at this point - the majority of the Big Six have now announced price rises happening within the coming weeks," she said.
"ScottishPower's rise is only the latest of the stinging hikes eating into the pay cheques of people across the UK and this is the biggest price rise so far from a major supplier."
ScottishPower booked a 53% drop in annual retail supply earnings to 139.1 million euro (£121.9 million) last year as customer numbers fell by 200,000 to 5.1 million.
The firm pinned the blame on higher costs hitting UK electricity margins, while its gas arm struggled in response to warmer weather in 2017.