Bob Woodward, the Washington Post reporter who broke the Watergate story that brought down President Richard Nixon, testified before the special prosecutor investigating the outing of a covert Central Intelligence Agency operative.

In the latest twist in an investigation that has brought indictments against Lewis “Scooter” Libby, the former chief of staff to Vice-President Dick Cheney, Mr Woodward testified that he was given the name of the agent, Valerie Plame, a month before she was publicly identified by columnist Robert Novak.

The revelation could threaten the case against Mr Libby assembled by Patrick Fitzgerald, the special prosecutor, according to legal experts. It could help bolster the claim by Mr Libby that he first heard about Ms Plame’s identity from other reporters, not from official government sources. It may also call into question whether she was identified deliberately or inadvertently.

In a statement released on Tuesday night and published in Wednesday’s Washington Post, Mr Woodward said he was told by a government official that Ms Plame, the wife of ambassador Joseph Wilson, “worked for the CIA on weapons of mass destruction as a WMD analyst”.

“I testified that the reference seemed to me to be casual and offhand, and that it did not appear to me to be either classified or sensitive,” he said. “I testified that, according to my understanding, an analyst in the CIA is not normally an undercover position.”

Ms Plame had been a CIA covert operative posted abroad but, at the time of Mr Novak’s column in July 2003, she was at CIA headquarters in Langley, Virginia.

Mr Libby has been charged with five counts of perjury and making false statements to the grand jury investigating the case. The charges arose out of an investigation into whether Ms Plame was deliberately identified as retaliation against Mr Wilson, who publicly charged the White House with distorting intelligence in the run-up to the Iraq war.

The investigation is continuing into the role played by Karl Rove, the White House deputy chief of staff.

Scott Fredericksen, a criminal defence attorney at Foley & Lardner in Washington, said the revelation “puts the prosecutor on the defensive.” He said it could make it easier for Mr Libby to persuade a jury that his version of events was plausible.

Mr Woodward testified after an unnamed government official informed Mr Fitzgerald earlier this month that he had discussed the issue with Mr Woodward. The journalist was at the time conducting a series of anonymous interviews with top Bush administration officials for a book on the Iraq war.

Mr Woodward testified that while he interviewed Mr Libby on June 27 2003 – several weeks before Ms Plame was publicly identified – he believed there had been no discussion of Mr Wilson or his wife. He also had conversations with Andrew Card, White House chief of staff, that did not involve Ms Plame, Washington Post executive editor Len Downie said on Wednesday. Mr Woodward has not revealed publicly which official gave him the name of the agent, though he did disclose the name to Mr Fitzgerald during his testimony.

Mr Woodward has been criticised by some other reporters for saying publicly that the scandal over Ms Plame had been overblown.

He told CNN last month that “when the story comes out, I’m quite confident that we’re going to find out it started kind of as gossip, as chatter.”

He called Mr Fitzgerald “a junkyard dog prosecutor” who turns over every rock, adding: “There’s a lot of innocent actions in all of this.”

Mr Woodward on Wednesday apologised for not previously telling Mr Downie or other senior Post editors of his involvement in the issue. “I apologised because I should have told him about this much sooner,’’ Woodward was quoted saying on the Post website. ”I explained in detail that I was trying to protect my sources. That’s job number one in a case like this.’”

“I hunkered down. I’m in the habit of keeping secrets. I didn’t want anything out there that was going to get me subpoenaed.”

Mr Downie said on Wednesday that he had accepted the apology and that no action would be taken against Mr Woodward, the paper’s assistant managing editor and one of the luminaries of American journalism. Judith Miller, the New York Times reporter who spent 85 days in jail for refusing to reveal her sources to Mr Fitzgerald, retired from the newspaper earlier this month following disagreements with her editors over her role in the controversy.

While Mr Woodward has reported on many of the biggest and most sensitive Washington stories of the past three decades, he said that Monday was the first time he had ever testified before a grand jury.

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