A "shadow government" Cold War plan in place since Dwight D. Eisenhower was president has been activated by the Bush administration.

Seventy-five or more senior officials live and work secretly away from their families in one of two fortified locations along the East Coast.

Members of the government-in-waiting, drawn from every Cabinet department and some independent agencies, are rotated out of Washington to the underground bunkers in case the nation's capital is attacked by terrorists, a senior government official said Thursday night.

The official, speaking on condition of anonymity, said the operation has been in effect since the first hours after the Sept. 11 terrorist attacks but has evolved over time and now is a permanent feature.

Without confirming details of the program, Bush told reporters in Iowa: "We take the continuity of government issue seriously because our nation was under attack. And I still take the threats we receive from Al Qaeda killers and terrorists very seriously."

"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 there is an ongoing government," Bush said. "That is one reason why the vice president was going to undisclosed locations. This is serious business. And we take it seriously."

Originally designed to help the government withstand Cold War nuclear threats, the shadow government plan was activated out of heightened fears that the Al Qaeda terrorist network might obtain a portable nuclear weapon. U.S. intelligence has no specific knowledge of such a weapon, but the risk was great enough to warrant the plan's activation, the official said.

The Washington Post, which first reported the classified "Continuity of Operations Plan," said the first rotations were made in late October or early November, a fact confirmed by a senior government official late Thursday.

The shadow government has sent home most of the first wave of deployed personnel, replacing them most commonly at 90-day intervals.

A government official who spoke to The Associated Press said the groups usually number 70 to 150 people, depending on the level of threat detected by U.S. intelligence. He said Bush does not foresee ever needing to turn over government functions to the secret operation, but believed it was prudent to implement the long-standing plan in light of the increasing war on terrorism and persistent threats of future attacks.

The underground government would seek to prevent the collapse of essential government functions in the event of a disabling blow to Washington, the official said. The officials come only from the executive branch.  The military also has its own facilities, and Congress and the Judiciary have "continuity of operations plans", but they are not set up for 24-hour operations.

The executive branch team would try to contain disruptions of the nation's food and water supplies, transportation links, energy and telecommunications networks, public health and civil order, the Post reported. Later, it would begin to reconstitute the government.

The government-in-waiting is an extension of a policy that has kept Vice President Dick Cheney in secure undisclosed locations away from Washington. Cheney has moved in and out of public view as threat levels have fluctuated.

As next in line to power behind President Bush, he would need help running the government in a worst-case scenario.

"We take this issue extraordinarily seriously, and are committed to doing as thorough a job as possible to ensure the ongoing operations of the federal government," Joseph W. Hagin, White House deputy chief of staff, told the Post, though he declined to discuss details. "In the case of the use of a weapon of mass destruction, the federal government would be able to do its job and continue to provide key services and respond."

According to the Post, the backup government consists generally of officials from top career ranks, from GS-14 and GS-15 to members of the Senior Executive Service. The White House is represented by a "senior-level presence," one official said, but well below such Cabinet-ranked advisers as Chief of Staff Andrew H. Card Jr. and National Security Adviser Condoleezza Rice.

Many departments, including Justice and Treasury, have completed plans to delegate statutory powers to officials who would not normally exercise them, the Post said. Others do not need to make such legal transfers, or are holding them in reserve.

The report said civilians deployed for the operation are not allowed to take their families and may not tell anyone where they are going or why. Their departures are labeled "business trips."

The two sites of the shadow government make use of local geological features to render them highly secure, the Post said. They are well stocked with food, water, medicine and other consumable supplies, and are capable of generating their own power.

The Associated Press contributed to this report.