President Reagan has approved a seven-year, $4.2 billion program to relocate up to two-thirds of the American public in case of imminent threat of a nuclear war, a U.S. agency announced yesterday.

The new program, made public by the Federal Emergency Management Agency (FEMA), which would be reponsible for carrying it out, represents a large-scale increase in government activity to prepare U.S. civilians to survive an atomic holocaust. Only $133 million annually is being spent for this purpose under existing plans.

FEMA spokesman James Holton said the new plans are based on a government assessment that a surprise nuclear attack on the United States is less likely than "a general exchange that would come out of a period of heightened international tension" with the Soviet Union. Holton said the working assumption is that the president would have at least a week's advance notice to move Americans from 380 "high-risk areas" to an unspecified number of "host areas" throughout the country.

The "high-risk areas" for a nuclear exchange, according to Holton, include 61 "counterforce targets" such as U.S. missile fields, strategic bomber bases and ports for nuclear submarines, and 319 cities--essentially all U.S. cities over 50,000.

Designation of the "host areas" where Americans could find shelter from radioactive nuclear fallout under the plan is being left to the states, Holton said.

It is not anticipated at this point that the fallout shelter areas would be stocked with food, as in the abortive Kennedy administration program of the early 1960s, the last time that the U.S. government contemplated large-scale civil defense operations and expenditures. Some of the "host areas" probably would have readily available food supplies, and "we might ask people to take food with them," Holton said.

There was no explanation from the White House, which arranged for the program to be announced by the little known FEMA, about why Reagan had decided on a large-scale upgrading of emergency preparations at this time. However, Reagan and other administration figures have expressed the view that the Soviet Union is better prepared for a nuclear war than is the United States, and some have stated that even an all-out nuclear exchange could be "survivable" with advance preparations.

One of the four objectives in the National Security Decision Directive signed by Reagan and made public yesterday is to provide for the survival of "a substantial portion" of the American public and government continuity in nuclear war.

No figures were attached, but Holton said the plan's objective after the seven-year effort is to double the number of Americans who would survive a nuclear attack. Since present calculations are that about 40 percent would survive a full-scale attack, he said, the objective is an 80 percent survival rate.

The other three objectives in Reagan's order are enhancement of strategic "deterrence and stability," including maintenance of "perceptions" that the international strategic balance is favorable to the United States; reduction of the possibility that the United States could be "coerced" in a crisis, and improvement of the government's ability to deal with natural disasters and other unspecified "large-scale domestic emergencies."

Reagan is reported to have approved a greatly expanded civil defense program in principle during a National Security Council meeting last Dec. 3. The Office of Management and Budget reportedly opposed the plan, arguing that it would cost far more than officially estimated.

The seven-year program, which Holton said has been priced by FEMA at $4.2 billion, calls for planning and "deployment" in countryside areas of such evacuation support facilities for civilians as communication systems, radiation detectors, organization for medical support and food distribution. This should make possible "a resuscitation of life" after a nuclear attack, Holton said.

The program also envisions construction of blast shelters for "key industrial workers" who would remain in high-risk areas during a nuclear crisis and unspecified protection of "key defense and population relocation support industries." As now approved by Reagan, the plan does not including major funding for these purposes.