In his first public remarks on the affair, Mr Bush urged anyone with information to come forward, saying: "I want to know the truth."

It is alleged that administration officials leaked the name of the agent - the wife of a former US diplomat - in revenge for his claims that intelligence officials exaggerated the case for war in Iraq.

Correspondents say it is extremely rare for the Department of Justice to conduct a full investigation into the alleged leaking of classified information.

Disclosing a covert agent's name is a criminal offence in the US, punishable by up to 10 years in prison.

White House staff were notified by e-mail on Tuesday that the Justice Department had decided to move from a preliminary investigation to a full-blown inquiry.

Staff were told to "preserve all materials that might in any way be related to the department's investigation".

These are likely to include telephone logs, e-mails, notes and other documents.

Speaking in Chicago, President Bush said he had told his staff to co-operate with the inquiry.

"This investigation is a good thing," he said.

"If there is a leak out of my administration, I want to know who it is. If the person has violated law, that person will be taken care of."

The allegations centre around the naming of Valerie Plame, wife of Joseph Wilson, the former US charge d'affaires in Baghdad.