Another 2.3 million households are facing higher energy costs after Scottish Power 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 £1,271.
SSE, which trades as Southern Electric, Swalec and Scottish Hydro, increased its tariffs by 9% today, while British Gas and npower announced plans on Friday to increase their charges.
Scottish Power 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."
His announcement comes despite Chancellor George Osborne calling on energy firms to reconsider the price rises that have sparked warnings of more households being plunged into a "long, cold winter".
Adam Scorer, director of energy at Consumer Focus, said the rush of price hikes from suppliers will fuel consumer concerns about the energy market.
He added: "No one welcomes a price rise. But when half the market raises prices in just four days, consumers will question whether this is in response to volatile commodity costs or simply pack behaviour."
The price increase for Scottish Power's direct debit customers will be 8.7%, compared with the 1.4% rise for quarterly cash and cheque customers. Around 700,000 households will not see an increase as they are on one of the company's fixed price or capped tariffs.
EDF has yet to make an announcement about prices, while E.ON has guaranteed a price freeze for 2012.
Scottish Power reported a 34% rise in the cost of delivering Government schemes this year, including energy efficiency programmes designed to reduce greenhouse gas emissions and in delivering ways to keep homes warm for less.
Other increases include an 11% rise in the cost of transporting gas and electricity and an 8% increase in wholesale energy costs.