Scottish & Southern Energy has become the latest of the "big six" utility groups to warn of higher tariffs, saying it will increase household electricity prices by an average of 11% and household gas prices by 18% from 14 September.The company, which has more than 10m customers in Scotland and central and southern England, said prices are going up due to rising energy network costs, mandatory environmental schemes and rising wholesale gas and electricity prices.Earlier this month, British Gas, which is owned by Centrica, said it was upping domestic gas and electricity prices by an average of 18% and 16% respectively from 18 August. Scottish Power kicked of the latest round of price rises, saying in June that it will increase gas prices by 19% and electricity prices by 10%."I am sorry that we have had to announce an increase in household energy prices at a time when many people's budgets are under strain, but the upward pressures on prices have become too great," said SSE chief executive Ian Marchant. "We want to avoid another increase in household energy prices and they won't go up again, if they have to, before August next year at the earliest."SSE also updated on current trading. It won 70,000 new gas and electricity customers in Britain and Ireland in the three months to 30 June, but electricity and gas consumption was down due to the unseasonably warm weather in spring.