ScottishPower increases the cost of its standard variable gas and electricity tariff by 3.7%, which further increases the pressure on compressed households.
The energy company said the changes will take effect on 8 October and will hit 900,000 customers with an average £ 46 added to annual invoices.
It blamed "persistent increases in the energy costs of wholesale trade".
Neil Clitheroe, chief executive of ScottishPower's retail department, said: "Since April we have seen the energy costs of the wholesalers rise significantly, and like others in the industry, this means we have to raise our prices.
"More than two-thirds of our customers have products with a fixed price or other rates that are not affected by this price change.
"Contact will be made with the customers who are dealing with the price change and offered the opportunity to switch to an alternative tariff at a fixed price and avoid this increase."
This is the second time this year that the Big Six energy company, owned by the Spanish multinational Iberdrola, has higher prices.
The company increased its standard variable gas and electricity prices for about one-third of its customers from June 1, with 950,000 households with an average increase of 5.5%.
The latest walk comes just a few weeks after British Gas said it is increasing its standard variable rate by 3.8%.
Energy regulator Ofgem has also recently increased its price ceiling for variable rates as a result of rising wholesale prices.
The indemnity rate, which protects five million households against overloading, will rise to £ 1,136 in October by £ 47 per year.
The watchdog said it raised the cap due to rising oil prices, which have pushed through to the gas wholesale prices.
Consumers can hardly keep up with the tsunami of price rises this year. This second increase in the Scottish Power price is another bitter pill to swallow for millions of households
Victoria Arrington, energy helpline
Various energy companies have increased their prices during the summer, affecting millions of households.
E.ON, SSE, Npower, EDF and Bulb all walked bills and accused the wholesale costs of the increases.
Consumer groups have alerted energy customers to rising rates and said they need to switch to a better deal.
Victoria Arrington, an energyhelpline spokesperson, said: "Consumers can hardly keep up with the tsunami of price rises this year – this second Scottish Power price increase is still a bitter pill to swallow for millions of households."