I have an obligation as the president to ... put measures in place that, should somebody be successful in attacking Washington DC, there's an ongoing government

George W Bush

The operation - officials said - was activated after the 11 September attacks but planning dates back to the Cold War era.

But senior congressional leaders have been quoted by the Washington Post newspaper as saying they had not been informed about the operation.

Senate Majority Leader Tom Daschle said he had not been told about the role, location or even the existence of the shadow government.

And an aide to House Minority Leader Richard Gephardt told the newspaper that he was also unaware of the administration's move.

The Washington Post said the shadow government involves 70 or more officials drawn from all departments - depending on the perceived level of threat.

Those taking part live and work underground for long spells at secret fortified locations on the East Coast.

The core group of federal managers would put into effect orders from the president or his constitutional successor.

America 'not safe'

Mr Bush said the threat of a catastrophic attack still existed as long as there was a terrorist threat.

Vice President Dick Cheney is said to be a major force behind the plan

"We take the continuity-of-government issue seriously because our nation was under attack," he said.

"And I still take the threats we receive from al-Qaeda killers and terrorists very seriously."

"Until this country has routed out terrorists wherever they try to hide, we're not safe," he added.

"I have an obligation as the president, and my administration has an obligation to the American people to ... put measures in place that, should somebody be successful in attacking Washington DC, there's an ongoing government."

"Bunker duty"

The plan for a shadow government has been implemented because of heightened fears that Osama Bin Laden's al-Qaeda network might obtain a portable nuclear weapon.

Officials fear Bin Laden's network may have nuclear weapons

An unnamed senior official told Associated Press news agency that, while US intelligence had no specific knowledge of such a weapon, the risk was great enough to warrant activation of the plan.

Other officials said the back-up government consisted of anything from 70 to 150 people at two principal locations on the East Coast.

Once activated for what some call "bunker duty", they lived and worked underground 24 hours a day, away from their families.

Those deployed on the operation are not allowed to tell anyone where they are going or why. "They're on a 'business trip', that's all," one official was quoted as saying.

Out of touch

Although it does not identify the sites, the Washington Post says they make use of geological features to render them highly secure.

They are well stocked with food, water, medicine and other supplies and are capable of generating their own power.

However, in their first significant operational use, managers discovered that computers were "several generations" behind current models and could not link to government databases.

There were also too few telephone lines and secure audio and video links to the rest of government.