Lewis "Scooter" Libby, the former chief of staff to Vice President Dick Cheney, pleaded not guilty today to charges relating to the unmasking of an undercover CIA agent.

Appearing on crutches because of a broken bone in his foot, Mr Libby listened to the charges against him - he is accused of perjury, obstruction of justice and lying to the FBI - and told US District Judge Reggie Walton: "With respect, your honor, I plead not guilty."

Mr Libby, 55, faces a possible 30-year prison sentence if he is convicted on all charges. His determination to fight the charges against him, which relate directly to the Bush Administration's case for the war in Iraq, has raised the possibility of a trial in which several members of President Bush's cabinet, including Mr Cheney, could be called as witnesses.

In legal documents before today's ten-minute hearing in Washington, it was revealed that Mr Libby has hired a team of celebrated trial lawyers to defend him, including Theodore Wells, an experienced Washington defence lawyer, and William Jeffress, a white-collar crime expert.

Mr Libby's next court appearance will take place on February 3.

Mr Libby was indicted last Friday by Special Counsel Patrick Fitzgerald, who has led a 22-month investigation into the leaking of the identity of Valerie Plame, an CIA operative whose husband, a former diplomat, accused the White House of twisting intelligence in its attempts to justify the invasion of Iraq.

Under American law it is a crime to knowingly disclose the identity of an undercover agent.

The Libby indictment alleges that Mr Libby, a former Pentagon official and Mr Bush's top foreign policy adviser, learned the identity of Ms Plame from Mr Cheney, his immediate superior, the CIA and State Department before sharing it with journalists during the summer of 2003.

Last week, Mr Fitzgerald said that Karl Rove, Mr Bush's chief political strategist who also discussed Ms Plame's identity with journalists, remained under investigation for his part in the affair.

The impending trial of Mr Libby will cast an unwelcome and slow-moving shadow over the Bush White House. Yesterday, Senator Trent Lott of Mississippi, a former Republican Senate Leader, questioned whether Mr Rove should continue in his role while he was under investigation.

"He has been very successful, very effective in the political arena. The question is, should he be the deputy chief of staff for policy under the current circumstances?" Senator Lott said on American television.

William Niskanen, chairman of the Cato Institute, the leading conservative think tank, and a top economic adviser to former President Ronald Reagan also suggested a shake-up: "Bush is going to have to sacrifice people who have worked with him to regain some initiative," he said.

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