In a revelation that places Mr Cheney at the centre of the scandal, it emerged that Lewis "Scooter" Libby, Mr Cheney’s Chief of Staff and the White House aide most likely to be indicted in the affair, first learnt about the existence of the CIA agent from the Vice-President weeks before her identity became public.

Notes of a previously undisclosed conversation between Mr Cheney and Mr Libby on June 12, 2003, leaked to The New York Times, contradict Mr Libby’s grand jury testimony that he first learnt about the CIA official from journalists. Instead Mr Libby’s notes show that he was told about Valerie Plame by Mr Cheney, and that the Vice-President knew that she worked at the CIA more than a month before she was identified in an article by the conservative columnist Robert Novak.

News of Mr Cheney’s involvement came amid fevered speculation in Washington that Mr Libby and Karl Rove, Mr Bush’s chief adviser, were to be indicted for their role in the affair, possibly today.

Patrick Fitzgerald, the special prosecutor leading what has been a two-year investigation, is due to wind it up on Friday. The grand jury convenes for its penultimate session today, and legal analysts believe that if he is bringing charges they will come this week.

Mr Libby’s notes show that Mr Cheney obtained his information about Ms Plame from George Tenet, then the CIA director, in response to questions about Joseph Wilson, who is Ms Plame’s husband and a former ambassador. Mr Wilson had accused the Bush Administration of twisting prewar intelligence to justify the invasion of Iraq.

The notes suggest that Mr Libby may have tried to steer investigators away from any involvement by Mr Cheney. There was no indication that Mr Cheney knew the agent’s name, and lawyers say Mr Fitzgerald has no plans to indict him. Mr Tenet is also not a focus of the investigation.

But yesterday’s disclosures will weigh on the White House as pressure grows on Mr Bush over his Supreme Court nomination of Harriet Miers, his former personal lawyer, and the rising death toll of US troops in Iraq.

The Plame scandal has its roots in a CIA-sponsored trip that Mr Wilson made to Niger in 2002 to investigate reports that Iraq had tried to buy uranium there. By June 2003 Mr Wilson was briefing journalists anonymously, disputing White House claims of an Iraq/Niger connection. On June 12, the day that Mr Cheney talked to Mr Libby, a story in The Washington Post cast doubt on the White House claims.

By then Mr Wilson had appeared on the radar of Mr Cheney, whose office was embroiled in a dispute with CIA factions over prewar intelligence. In July 2003 Mr Wilson went public with his claims. A week later Mr Novak disclosed the identity of Ms Plame.

Mr Wilson said that his wife was an undercover CIA agent, and that White House aides had leaked her identity as retribution. Mr Bush appointed Mr Fitzgerald to investigate.

No evidence has emerged publicly to suggest that Mr Rove or Mr Libby knew Ms Plame’s alleged covert status. But it has become clear that the pair spoke to journalists about her before Mr Novak’s column.

Parts of their grand jury testimony have contradicted the reporters’ accounts, heightening speculation that Mr Fitzgerald may bring perjury or obstruction of justice charges.

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