Air traffic likely will double early in the next century and billions of dollars in airport expansion and thousand-seat planes may be needed to prevent gridlock in the skies, said a report released Saturday.

"Delays in air travel have been mounting and could reach staggering proportions in the coming years," the National Research Council concluded in its report on a one-year study conducted for the Federal Aviation Administration.In 1987, each of 21 major airports experienced more than 20,000 hours of flight delays. But by 1997, the report said, 39 airports could reach that level. Delays could reach 100,000 hours a year at such major hubs as Chicago, Atlanta and Denver.

The nation's air transport system is expected to carry about 1.3 million domestic and international passengers a day in 1990.

But shortly after the year 2000, the number is expected to reach 2.5 million a day or nearly 1 billion passengers a year, the report said.

"If this growth continues, the system could be carrying 4 million to 5 million passengers daily by 2040, more than triple the present volume of traffic," the report said.

If plans are not made now to handle such vastly increased levels of air travel, "congestion and delay . . . will be a constraint on growth that will profoundly affect the society in the 21st century."

The study calls for a blend of new construction, changes in approach and greatly expanded research and development.

Specific recommendations include:

-The FAA should establish a strategic planning process through the year 2040 with shorter-range goals in each 10-year period.

-Increase capacity at up to 50 existing airports at a cost of $40 million to $65 million each, with runway changes and improvements in air-traffic control.

-Develop new secondary hubs for connecting flights at some or all of 28 under-used airports at a cost of $250 million to $500 million each.

-Study ways to improve airport design, manage resources, reduce airport noise, and integrate air and land transportation.

-Create an expanded, centrally organized airport system by giving the federal government authority to oversee improvements in airport capacity, possibly including construction of 10 new major airports. This could involve federal costs of $50 billion to $90 billion over the next 40 years.

-Seek market-based solutions to capacity problems. This strategy also involves the construction of 10 new airports but would give state and local authorities responsibility for increasing airport capacity. Estimated federal financing: $38 billion to $75 billion.

-Develop larger subsonic jets of up to 1,000 seats for heavily traveled routes, supersonic commercial transports for international routes, and short-haul aircraft and miniports at suburban locations for transfer purposes. Develop new surface transport systems to absorb some of the 200-to-500 mile intercity traffic now handled by aircraft.