Pan American World Airways today announced plans to create air shuttle service between Washington National Airport and Newark International Airport.

At the same time Pan Am said it would seek an "international department facility" at National Airport to connect passengers with its overseas flights in New York and Miami.

The announcements came as Pan Am merged with National Airlines making it one of the nation's largest air carriers. Pan Am Chairman William T. Seawell said the merger would make Pan Am the fourth largest airline behind United, Trans World and American Airlines.

Pan Am had pledged the shuttle service last year when it made its merger proposal before the Civil Aeronautics Board.

Seawell said the air carrier will soon "ask New Jersey officials to arrange talks on how best to move ahead" in setting up the Newark-Washington shuttle. A spokesman for Pan Am said the proposed shuttle would operate every other hour from 7 a.m. until 9 p.m. and, like the Eastern shuttle, would have back-up planes to guarantee every passenger a seat.

James Wilding, director of Metropolitan Washington Airports for the Federal Aviation Administration, said that Pan Am might have trouble finding enough slots at crowded National Airport to set up a shuttle service. fFurthermore, he noted, the number of slots at National Airport soon will be reduced from 40 an hour to 36 an hour.

The FAA does not determine which airlines get the slots -- a committee of the airlines themselves carve those up.

Eastern once operated a shuttle between National Airport and Newark but cancelled it in 1973 as part of a general route cutback during the Arab oil embargo. Eastern does operated several scheduled flights to Newark every day.

Eastern carried about 1.5 million persons between LaGuardia and National airports last year, nearly 8 percent more than in 1978.

Pan Am has long wanted domestic routes to feed passengers into its vast network of international flights. It waged a protracted struggle with Eastern Airlines and Texas International Airlines over control of National Airlines. The CAB approved both Texas International and Pan Am's merger proposals, but turned down Eastern's bid because National and Eastern overlap on many north-south routes.

In creating an international departure facility at National Airport, Pan Am said it would "give passengers a convenient alternative to Dulles Airport for international travel."

Wilding of the FAA said that Pan Am has not discussed the proposal with him.

"National (Airlines) has had a number of flights to (New York's international airport) John F. Kennedy that some folks travelling on international flights find convenient," Wilding said.

Wilding said there probably would be little trouble if all Pan Am wants to do is "relabel what already exists. But if they want to try to market National Airport as a long-haul airport, we'd want to talk to them a little more seriously."

In outlining the merger plans today, Seawell said that National will be operated as a wholly owned subsidiary until the personnel, the routes and the equipment of the two airlines can be meshed.

Pan Am Vice President William Roy said the biggest hurdle that must be overcome is labor. Pan Am has 28,200 employes and National 8,352.

In many cases employes doing identical jobs for the two airlines are represented by different unions -- National flight attendants, for example, belong to the Transport Workers' Union while Pan Am's are members of the Independent Union of Flight Attendants. Pan Am has said it will recognize the unions that now represent Pan Am employes, but other unions may seek representation rights.

Furthermore, as in the case of pilots, seniority and recall rights must be worked out. Several hundred pilots at Pan Am are now laid off, but some of them have more seniority with Pan Am than do some National pilots who are now flying.

Ray said that because of complexities in merging the two labor forces, "summer is the earliest we could plan on announcing a merged airline image from the customers' standpoint."

Seawell said it would probably be late summer or early fall before the consolidation is fully effective.