The State Department, struggling to cope with severe budget restrictions, is preparing the largest round of job cuts in the department's modern history. About 1,270 jobs, or about 8 percent of its 15,800 Foreign Service and Civil Service positions, will be eliminated.

Senior officials said yesterday that the proposed cuts probably will require the department to ask Congress for legislation to permit early retirement incentives and a follow-up reduction in force (RIF) because neither is allowed by the Foreign Service Act of 1980.

The officials said that they and other senior officials believe the cuts will have what one called "a devastating effect" on morale and will severely impede the department's performance.

The sources charged that the cuts had been planned in secrecy without consulting with the various bureaus of the department and, as a result, reflect what one called "a numbers-crunching approach that concentrates on eliminating people rather than expenditures for things such as buildings and equipment that could have been deferred . . . ."

In particular, the sources said, many senior officials think that the department should have sought to save money by postponing plans for construction of a new campus in suburban Virginia for the Foreign Service Institute, various embassy construction projects overseas and extension of its computer operations.

The department plans to close two small African embassies -- in Equatorial Guinea and the Comoros Islands -- and 13 consulates. Although the list of consulates still is not firm, those tentatively slated for closure are Bordeaux, France; Porto Alegre, Brazil; Brisbane, Australia; Auckland, New Zealand; Oran, Algeria; Oporto, Portugal; Edinburgh, Scotland; Quebec, Canada; Zurich, Switzerland; Antwerp, Belgium; Adana, Turkey; Surabaya, Indonesia, and Kaduna, Nigeria.

The cuts are outlined in a closely held reorganization plan prepared under the guidance of Deputy Secretary of State John C. Whitehead. The Washington Post has obtained a copy of the plan, which is supposed to be in final form by Saturday in order to deal with an anticipated $84 million shortfall in congressional appropriations for the State Department during fiscal 1988.

The plan envisions a series of job and internal office consolidations, reduced operations and other cost-cutting measures. It would eliminate slightly more than 200 positions overseas, but most of the job cuts are scheduled to involve the department's Foggy Bottom headquarters. Although both Foreign Service and Civil Service personnel will be affected, the nature of the proposed consolidations indicates that the cuts will fall most heavily on the Foreign Service.

In one instance, 55 jobs would be eliminated by merging the Bureau of International Communications Policy into the Bureau of Oceans and International Environmental and Scientific Affairs. Other bureaus that deal with press and public information, personnel, congressional relations, economic and political-military affairs would face personnel cuts, in some cases as much as 25 percent.

Additional planned cuts would eliminate the Latin American bureau's public diplomacy office, which has become a center of controversy because of its role in promoting the cause of the Nicaraguan contras; eliminate the staff of the ambassador to the Organization of American States by effectively making the position part of the Latin American bureau, and reduce the size of the policy planning staff.

The plan emphasizes "eliminating overlapping and duplicative activities" by cutting officials from various geographic bureaus that deal on a regional basis with subjects such as economics and political-military affairs.

One senior official, noting that the cuts would eliminate as many as 60 Foreign Service officers specializing in economic affairs, said that will impair the department's economic reporting and analysis capabilities to such a degree that Secretary of State George P. Shultz, an economist himself, will have to call on the Treasury Department or the Central Intelligence Agency for detailed economic briefings in many areas.

Other measures include eliminating "superior performance pay" for members of the Senior Foreign Service, abolishing the 10 percent pay differential for "hardship posts," ending incentive pay for officers who achieve proficiency in critically needed difficult languages and restructuring the Foreign Service health insurance program.