The Department of Transportation is likely to propose reducing the number of passengers allowed to use National Airport from 16 million a year to about 15 million, according to government and aviation industry sources.

Airline companies are opposed to any passenger reduction as an unfair restriction of their business. Civic groups that have campaigned for years to divert traffic from the busy airport to Dulles International Airport would see the cut as a major victory.

The proposal, said to be under study by top DOT officials, comes while the department is considering an Eastern Airlines request to introduce larger jets to the airport.

Some industry analysts suggested yesterday that the department might first propose the passenger reduction to help dampen civic protests that are likely to occur when the new Boeing 757 arrives for tests at National.

Transportation Department spokesman Tom Blank yesterday said traffic policies at National are under review, but he said that any reduction in the passenger cap would not be related to the 757 decision. "No senior officials of the department have made any decisions on any changes or proposed changes," Blank said.

Cutting the passenger cap would not reduce the airlines' current business at National, which was used by about 13.3 million people in 1982. It would slow future growth there and encourage expanded service at Dulles and Baltimore-Washington International Airport.

The 16-million passenger cap is contained in a formal policy on noise and congestion at National that the Federal Aviation Administration implemented in 1981 after years of delicate negotiations between Congress, the airlines and civic groups. The plan also reduced flights slightly and established noise limits.

Aviation sources generally said yesterday that while no final decisions have been made, department officials have indicated in meetings and telephone calls that they are leaning toward lowering the cap to about 15 million, with the figure of 14.8 million being mentioned specifically. Some said a formal announcement is expected within two weeks.

The Air Transport Association, which represents major airlines, last month sent a telegram to Transportation Secretary Elizabeth Dole saying "the airlines are strongly opposed to such a change." Airlines watch FAA policy at the federally owned National Airport closely, as an indicator of what may be coming for airports elsewhere in the country.

In 1982, passenger traffic at Dulles rose by 12 percent, while National's fell off by 6 percent. This apparently was due to heavy restrictions on traffic at National after the air traffic controllers' strike, and to expansion of service at Dulles.

Congressional support for reductions in the cap at National has been led by Rep. Frank R. Wolf (R-Va.), who last month urged Dole to lower the cap to between 14 and 15 million.

The FAA said a test flight into National by a 185-seat 757 scheduled for Saturday was postponed for at least two weeks to allow for further preparation. Eastern says the 757 is much quieter than jets now using National and that nearby residents would benefit from its use.

Wolf last week wrote to FAA chief J. Lynn Helms pointing out safety questions about the 757 raised by the Coalition on Airport Problems, an umbrella group of civic organizations, such as problems the twin-engine jet might encounter if one engine should fail on take-off.