The inquiry has already questioned the president's spokesman and one of his media advisers over the identification of Valerie Plame, which is developing into one of the administration's main headaches in an election year.

However, informed sources said last night that three of the five officials who are the real targets of the probe work or worked for Mr Cheney.

Until recently, President Bush has insisted that Mr Cheney would be his vice-presidential candidate in the November elections, despite his history of heart trouble.

But recent polls conducted by the White House have suggested that growing unpopularity of the taciturn ex-businessman and powerful administration hawk threatens to sink the president.

Mr Cheney is already under intense fire from Democrats for his personal role in shaping the case for war against Iraq, frequently visiting the CIA to question assessments that played down Saddam Hussein's arsenal.

His former role as head of a giant oil services corporation, Halliburton, is also under scrutiny, as the company is under investigation for bribery when Mr Cheney was in charge and, more recently for war-profiteering in Iraq.

But the grand jury investigation into the CIA leak is potentially the most explosive threat to his long-term political survival.

The case centres around the leaking to the press in July of the name of Valerie Plame, apparently in response to public questioning of the US case for war against Iraq by her husband, Joseph Wilson, a former ambassador.

The leaking of an undercover agent's identity is a serious crime under US law. The hearings are leading justice department investigators towards the vice president's office, according to a source familiar with the investigation.

"Three of the five people who are targets work or worked in Cheney's office," the source said.

He added that members of the defence policy board, a Pentagon advisory group, are also under scrutiny. Sensing the danger to the administration, the Democratic National Committee (DNC) chairman, Terry McCauliffe issued a statement to say: "Now that the FBI is getting closer to finding out who inside the Bush White House put the lives of CIA agents in danger, we hope that President Bush will keep his word and hold accountable those responsible for the White House leak - no matter how high their post."

The chief White House spokesman, Scott McClellan, outlined the president's position. "The leak of classified information is a very serious matter," he said

A parallel grand jury is looking into the forgery of a document that surfaced in Italy before the war, purporting to show Iraqi attempts to buy uranium in Niger. Despite doubts over its authenticity, the document underpinned US and British claims, since proved groundless, that Saddam was reconstituting his nuclear weapons programme.

A third grand jury in Washington is looking into allegations that a Halliburton subsidiary paid $180m in bribes to secure lucrative contracts to build a gas plant in Nigeria, at the time Mr Cheney was chief executive, from 1995 to 2000.

More recently the corporation has been caught overcharging millions of dollars for the delivery of petrol to the US military in Iraq.