Ofgem slapped a £2.5m fine on Centrica's British Gas after the UK energy regulator found that the company had failed to deal with customer complaints properly. An investigation led by Ofgem found that UK's largest household energy supplier failed to re-open unresolved customer complaints and did not provide customers whose complaint they could not resolve with some key details about the redress service provided by the Energy Ombudsman.Also, Ofgem concluded that British Gas had failed to put in place adequate processes and practices for dealing with complaints from micro-businesses. The regulator's probe into British Gas began in June 2010."Ofgem's action against British Gas is welcome," said Audrey Gallacher, director of energy at Consumer Focus."Frustrating the attempts of customers to get complaints sorted out is unacceptable." Among other inquiries, Ofgem is also currently investigating Npower and EDF Energy for complaint handling, Scottish Power, Scottish and Southern Energy, EDF Energy and Npower for mis-selling.Shares of Centrica dipped 1.2% to 321.9p in morning trading session in London. Rival Scottish and Southern Energy shares were the worst performing on the benchmark index.---AR