The Federal Aviation Administration has suspended use of the international air route that Korean Air Lines Flight 007 was supposed to be following when it strayed into Soviet airspace and apparently was shot down.

At 4:30 p.m. Thursday, about 26 hours after the plane went down, FAA Administrator J. Lynn Helms ordered the air traffic control center in Anchorage to deny all clearances to planes seeking to fly R20, which comes within 17 miles of Soviet airspace. The closest parallel route is about 67 miles from Soviet airspace.

When KAL Flight 007 and the 269 persons on board were lost, the plane had deviated from R20 by 310 miles, according to a Soviet statement yesterday. The reason for that major deviation, which is in line with American reports, remained a mystery.

Helms decided to suspend flights on R20 because "there is a need to clarify all the circumstances" of the incident, FAA spokesman Edmund Pinto said. He called the suspension "a precautionary measure."

Other Transportation Department officials said Helms took the action after meeting Thursday afternoon with Lawrence S. Eagleburger, undersecretary of state for political affairs.

With the suspension, four routes are available for flights between Anchorage and Tokyo, the shortest Great Circle corridor between North America and the Orient.

That could force some delays in North Pacific schedules during high traffic periods, experts said. However, both Northwest Orient Airlines and Pan American World Airways, which have many flights in the corridor, said yesterday they are maintaining schedules without difficulty.

Helms' action closes the route to all traffic because the FAA controls the northern entrance to R20. The decision was reached after consultation with the Japanese, who monitor by radio aircraft flying the southern half of the route.

The aircraft are responsible for maintaining course, however, and are not monitored by civilian radar for most of the route's length.

Airplanes are expected to maintain course by using a computer-controlled guidance device called an Inertial Navigation System (INS). The Korean airplane, the 186th Boeing 747 built, was equipped with an INS manufactured by Litton Industries and installed a year ago, a Litton spokesman said.

Speculation about how the plane went off course centered around the possibility of sloppy INS programming by the flight crew. The computer should have been fed a number of geographic locations the flight would follow en route; then it would guide the airplane along that course.

The route taken by Flight 007, according to one thoery, would result if the crew failed to enter intermediate points into the computer but included only the first geographic location on R20 and Seoul, the destination of the flight.

A Litton spokesman confirmed that the INS computer could accept only two entries, one for a starting point and one for a destination.

Retired Air Force general George Keegan said on CBS television that Korean crews "tend to be careless." Korean Air Lines officials in New York did not return telephone calls.

An official at the International Civil Aviation Organization (ICAO) in Montreal, which coordinated the effort to establish R20 as a protected international route, said there had been no complaints from the Soviets about anybody, Koreans or otherwise, straying from the path.

Nonetheless, aviation industry sources say that airliners of many nations have accidentally penetrated Soviet airspace on occasions in the past and that INS computers have been misprogrammed.

The Soviet Union and South Korea are both members of ICAO, as are the United States and most other nations.

South Korea formally has requested assistance from ICAO in obtaining information about what had happened to Flight 007, and ICAO has sent a query to Moscow. The ICAO's triennial session is to begin in Montreal Sept. 20 and could provide an international forum for full debate on the incident.

The most highly publicized previous incident of an aircraft penetrating Soviet airspace also involved a Korean flight. That was in 1978, when a Boeing 707 was forced by Soviet fighters to land south of Murmansk on a frozen lake. The plane was more than 1,000 miles off course. Two passengers were killed and 13 others injured.