ScottishPower said it will increase its standard domestic gas and electricity prices from March 31, 2017 — but nearly two out of three homes supplied by ScottishPower are on products or payment methods unaffected by the price change.

ScottishPower, a subsidiary of Basque group Iberdrola, said the price increases follow rises in energy wholesale markets and “compulsory non-energy costs” including costs for decarbonising electricity generation and an upgrade to smart meters.

Dual fuel prices will increase by an average of 7.8%.

Electricity prices will increase by an average of 10.8% and gas prices by an average of 4.7%.

About one third of ScottishPower customers — 1.1 million — will be affected by the change.

“Pre-payment customers are not affected by this announcement, but they will see separate changes as the CMA price cap is implemented,” said ScottishPower.

“ScottishPower has worked hard over recent years to move customers from standard prices and now has over half of its customers on fixed price deals – more than all other major suppliers.”

ScottishPower said it would continue to encourage customers to move to fixed price deals and would write to customers on standard tariffs with a personalised quote, showing how much could be saved.

“Customers will have at least 30 days to move to the best fixed price tariff that suits them, and avoid this increase,” said the firm.

ScottishPower said its customer service staff would stop sales activity for a week to help affected customers make sure they are on the best deal.

Colin McNeill, ScottishPower UK Retail Director, said: “This increase will apply to one in three of our customers, and we continue to work hard to move even more customers to our fixed price deals.

“We will be writing to all those affected, outlining the changes and encouraging more loyal customers to move to a deal that best suits them.

“This price change follows months of cost increases that have already led to significant rises in fixed price products that now unfortunately have to be reflected in standard prices.”