Washington ? A “shadow government” consisting of 75 or more senior officials has been living and working secretly outside Washington since Sept. 11 in case the nation’s capital is crippled by terrorist attack, government officials said Friday.

Such an operation was conceived as a Cold War precaution against nuclear attack during the Eisenhower administration but never used until now. It went into effect in the first hours after the terror attacks and has evolved over time, said senior government officials who provided details of the plan.

Without confirming details of the government-in-waiting, White House spokesman Scott McClellan said, “This is a responsibility we took seriously before Sept. 11 and after Sept. 11. We have a responsibility to the American people to take all appropriate steps possible to ensure continuity of government and we will continue to take that responsibility seriously.”

The shadow government plan was activated out of heightened fears that the al-Qaida terrorist network might obtain a portable nuclear weapon. U.S. intelligence has no specific knowledge the network has such a weapon, but the risk was great enough to warrant the activation of a plan, said a senior government official who spoke on condition of anonymity.

Under the classified “Continuity of Operations Plan,” which was first reported by The Washington Post in Friday editions, high-ranking officials representing their departments have begun rotating in and out of the assignment at one of two fortified locations along the East Coast.

The Post said the first rotations were made in late October or early November, a fact confirmed by the senior government official.

A second government official who has visited one of the secure sites in a mountainside outside Washington said it is equipped with telephones, television sets, command centers and computers. It is a large office space deep beneath the ground, sectioned off by agencies.

In an unsettling reminder of the stakes involved, the official recalled seeing food rations at the site.

Officials who are activated for the duty live and work underground 24 hours a day, away from their families. The shadow government has sent home most of the first wave of deployed personnel, replacing them most commonly at 90-day intervals, the Post reported.

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 President 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 gathering war on terrorism and persistent threats of future attacks.

The team, drawn from every Cabinet department and some independent agencies, would seek to prevent the collapse of essential government functions in the event of a disabling blow to Washington, the official said.

The underground government 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, at times to the same location that the senior government officials have been hidden, a government source said Friday.

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

The plan gelled in December when Bush issued a little-noticed executive order that spelled out the line of succession at several key federal agencies in case a Cabinet secretary is killed or incapacitated. In great detail, the document lists  in order of power  the top half dozen or so officials who would take over if an agency’s hierarchy was dismantled.

The shadow government is drawn from that deep pool of officials who are now formally part of the line of succession, sources said.

In addition, at least one Cabinet secretary is kept out of Washington at all times to help maintain the continuity of government, one government source said. For nearly two weeks at the end of December, for example, Attorney General John Ashcroft was secretly spirited to his farm in Missouri and kept under high security.

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.

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.