ScottishPower has become the latest energy company to announce a rise in gas and electricity prices for winter.Tariffs for 2.5m households will climb by an average of 2% for gas and by 8.9% for electricity, the power group said. Last week, British Gas (owned by Centrica) followed in the footsteps of Scottish and Southern in announcing price rises. Three of the 'big six' energy suppliers have now announced price hikes.Scottish Power said the price rise would add about £1 a week to the weekly cost of energy for a dual fuel customer paying monthly by direct debit.'The change in prices announced today is as a result of sustained increases in the wholesale energy market, with the wholesale costs for an average Dual Fuel customer up 26% since 31 March 2010,' said Raymond Jack, ScottishPower's director of energy retail.