The head of one of Britain’s main opposition political parties has resigned.Liberal Democrat Charles Kennedy said he would not stand in a forthcoming leadership election. “I am announcing today that when nominations do open for the leadership of the party, I shall not be putting my name forward,” he said. “I am standing down as leader with immediate effect.” The 46-year-old has been under increasing pressure since acknowledging a drink problem on Thursday. He spoke out shortly before a TV station was due to run an exposé. Matters came to a head when 25 of the party’s 62 members of parliament signed a statement saying they would not serve under him beyond the weekend.