Mr Cheney calls Mr Libby "one of the more honest men I know"

Vice-President Dick Cheney's former chief of staff, Lewis "Scooter" Libby, is accused of obstructing an investigation into the leak of a CIA agent's name.

The prosecution will attempt to show he lied to investigators about his role in the disclosure of Valerie Plame's identity.

His defence will claim he had no motive to do so - and is set to take the historic step of putting Mr Cheney on the stand.

There does not seem to be precedent for a sitting vice-president testifying in a criminal court.

Washington insiders are salivating at the thought of prosecutor Patrick Fitzgerald - a famously tough cross-examiner - harrying Mr Cheney, whose debating skills are legendary.

CIA revelation

Mr Fitzgerald began an investigation more than three years ago into who revealed Ms Plame's identity in the summer of 2003 to a number of top Washington journalists.

Her CIA connection was disclosed in a column by Robert Novak in July 2003.

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The column was published only eight days after her husband, former diplomat Joseph Wilson, wrote in the New York Times that a key justification for the US-led invasion of Iraq was unfounded.

Mr Wilson wrote that he had been sent to Niger to find out whether Saddam Hussein had tried to buy ingredients for nuclear weapons from the African country.

He said he had found no evidence of it - but that Mr Bush had ignored his findings and made the claim in a State of the Union address before the invasion.

Mr Wilson accused the White House of leaking Ms Plame's CIA connection to discredit him, or in revenge for his criticism.

The White House denies the allegation.

Mr Fitzgerald never charged anyone with the leak, but after two years of questioning key figures in the White House and the Washington media, he charged Mr Libby with lying to investigators about conversations he had with journalists.

Mr Libby resigned after being charged at the end of 2005, saying he was confident of being "completely and totally exonerated".

He faces up to 30 years in prison and a fine of up to $1.25m (£637,000) if he is convicted on all counts.

Source revealed

Richard Armitage, who was Colin Powell's deputy when Mr Powell was secretary of state, has since admitted that he was the original source of the leak to Mr Novak.

He said it was innocent gossip.

Ms Plame says the White House attacked her husband

Mr Libby's defence will claim that he was too busy with national security issues - he was Mr Cheney's national security adviser in addition to being chief of staff - to remember the details of his conversations with journalists months after they took place.

Mr Cheney called Mr Libby "one of the more honest men I know" in a television interview ahead of the trial.

The vice-president refused to say if he would testify in person or via videotape.

Experts say the prosecution's challenge will be to keep the trial focused tightly on what Mr Libby did or did not say to investigators, since Mr Armitage has been revealed as the original source of the leak.

The trial is expected to last about six weeks.