Another 2.3 million households were facing higher energy costs after ScottishPower announced its prices will rise by an average 7%.
The increase, which takes effect from December 3, means a typical monthly direct debit customer will see their average annual dual fuel bill rise to £1271.
SSE, which trades as Southern Electric, Swalec and Scottish Hydro, also increased its tariffs by 9% on Monday.
British Gas and npower announced plans on Friday to increase their charges.
ScottishPower blamed the price hike on increased costs, especially for transporting gas and electricity to customers' homes and the cost of energy efficiency programmes.
Neil Clitheroe, chief executive of retail and generation, said: "We work hard to protect our customers and we regret that we've had to announce a price increase today."