A New York Times reporter has been released after nearly three months in prison and will give evidence today in an inquiry into White House sleaze.

Judith Miller will appear before a grand jury trying to establish which member of President Bush's administration, in an act of revenge, leaked the identity of a woman CIA agent to the press.

Ms Miller was jailed in July for refusing to reveal her sources, but was released last night after her highly-placed White House contact voluntarily agreed that she could identify him.

The inquiry is trying to find out how Valerie Plame, an undercover CIA agent, came to be named on July 14 2003 in a comment piece by the New York Times columnist Robert Novak.

It has been strongly suggested that Ms Plame's name was leaked to the newspaper by White House aides to punish her husband, the former US ambassador Joseph Wilson, for publicly criticising the Bush administration over Iraq.

Ms Miller did not write the article naming Ms Plame, but - it now emerges - on July 8, less that a week before the article appeared, she did have conversations with Lewis "Scooter" Libby, who is US Vice President Dick Cheney's chief of staff, and one of the White House figures suspected as the source of the leak.

Ms Miller was freed from a federal detention centre outside Washington last night after Mr Libby said she could name him, thus freeing her to testify in the special prosecutor’s investigation into the case.

Special prosecutor Patrick Fitzgerald wants her to provide documents and testimony relating to conversations she had with Mr Libby. He has said in the past that once he secures Ms Miller's evidence he should be able to wrap up his inquiry fairly quickly.

The investigation has caused political damage to the Bush White House and could still result in criminal charges against government officials. Until a few months ago, the White House maintained for nearly two years that Mr Libby and presidential aide Karl Rove were not involved in leaking Plame’s identity.

The timing of Ambassador Wilson’s criticism was devastating for the White House, which was already on the defensive because no weapons of mass destruction had been found in Iraq. The president’s claims of such weapons were the main justification for going to war.

The CIA had sent Mr Wilson to Africa where he was unable to confirm an intelligence report that Iraq was purchasing yellowcake uranium from Niger. More than a year later, Wilson wrote a comment piece for The New York Times, "What I Didn’t Find In Africa," and asked the question: "Did the Bush administration manipulate intelligence about Saddam Hussein’s weapons programs to justify an invasion?"

Two days after the critical article appeared in the paper, Mr Libby spoke to Ms Miller. It was a few days after this meeting that the columnist Mr Novak wrote that two senior administration officials had told him that Ms Plame had suggested sending her husband to Niger on behalf of the CIA - thus outing Ms Plame as a CIA agent.

Time magazine reporter Matthew Cooper has testified recently that Mr Rove and Mr Libby had spoken to him about Ms Plame in the same week in July 2003 when Miller spoke to Libby.

In October 2003, with the criminal investigation gaining speed, White House spokesman Scott McClellan said of Rove and Libby: "Those individuals assured me they were not involved in this (leaking of Plame’s identity)."

Mr Bush has given varying accounts of the circumstances under which he would discipline those involved in the Plame probe. In September 2003, he said "we’ll take the appropriate action" and his spokesman said "they would no longer be in this administration."

In June 2004, Mr Bush reiterated the pledge, answering "yes" when asked if he would fire anyone in his administration who leaked Plame’s name. But in July, amid revelations that Rove and Libby had been involved in the leaks, Mr Bush's line had softened somewhat and he said that "if someone committed a crime" he would be fired.

Of the reporters swept up in Fitzgerald’s investigation, Ms Miller is the only one to go to jail. Mr Novak apparently has cooperated with prosecutors, though neither he nor his lawyer has said so.

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