Former White House spokesman Ari Fleischer has told a court that a key former aide to the vice-president revealed secret information to him.
Ari Fleischer has struck an immunity deal with prosecutors
Mr Fleischer said Lewis Libby told him on 7 July 2003 that Valerie Plame, the wife of administration critic Joseph Wilson, worked for the CIA.
Mr Libby is on trial, charged with lying to investigators trying to find out who leaked the secret information.
Mr Libby denies the charges. No-one was ever charged with the leak itself.
Mr Fleischer was a prosecution witness at the high-profile trial, becoming the fourth current or former official to contradict Mr Libby's account of how and when he learned Ms Plame worked for the CIA.
It is a crime to knowingly reveal the identity of an undercover CIA agent.
Mr Fleischer said Mr Libby told him about Ms Plame at lunch a day after her husband published an article accusing the Bush administration of misusing intelligence in the run-up to the war against Iraq.
Mr Libby added that the information was "hush-hush", Mr Fleischer testified.
If found guilty, Lewis "Scooter" Libby - who was Vice-President Dick Cheney's chief of staff - could face up to 30 years in jail.
'Attack on critic'
The trial in Washington is scrutinising the conduct of the White House in the run-up to the Iraq war.
Mr Wilson, the administration critic, alleges that his wife was "outed" in revenge for his attack on President George W Bush's use of intelligence to justify the war.
Lewis Libby is accused of perjury and obstruction of justice
Mr Libby told FBI agents and a grand jury that he learned Ms Plame was a CIA agent from reporters.
But during his testimony on Monday, Mr Fleischer - who was chief White House spokesman until mid-2003 - appeared to contradict this.
He said Mr Libby had invited him to lunch. The two discussed Mr Fleischer's planned departure from office and their shared interest in American football, the former press secretary added.
The conversation, he said, then turned to the growing criticism of the administration voiced by Mr Wilson.
"Ambassador Wilson was sent by his wife," Mr Fleischer recalled Mr Libby saying. "His wife works for the CIA."
Mr Fleischer testified under an immunity deal with prosecutors, which correspondents say he sought because he subsequently discussed Ms Plame with reporters.
Defence lawyers attacked his credibility, pressing him on how well he remembered the details of a conversation that took place more than three years ago.
Mr Fleischer responded that he was sure Mr Libby had told him about Mr Wilson's wife working for the CIA, though he admitted he could not be "absolutely certain" Mr Libby had used her name.
Last week other witnesses - including Mr Cheney's spokeswoman Cathie Martin - told the court Mr Libby had discussed Ms Plame before he had admitted doing so.
Mr Libby said he had not lied about the date he heard about Ms Plame's identity, but simply forgot previous conversations at a time when his priority was national security.