The former Chief of Staff to US Vice President Dick Cheney has pleaded innocent to charges stemming from an investigation into a CIA leak. Lewis Libby resigned last week and is facing serious charges including obstructing justice and perjury.Just hours before Libby brought in a top defence lawyer to take charge of the case. The identity of CIA operative Valerie Plame was leaked to the press after her diplomat husband accused the Bush administration of twisting intelligence to justify the war in Iraq. Top White House officials could now be called to testify at his trial. The US government’s use of pre-war intelligence on Iraq could also be put under the spotlight.