What critics of National Airport have wanted to do -- cut the number of flights there -- the airlines unintentionally have done themselves.

The reason is continued disagreement among the carriers about how to distribute 35 additional daily flights the government is willing to allow at the close-in airport.

The 35 extra flights would have brought National Airport's commercial flights up to the level allowed by the government's metropolitan airports policy. It has been reduced since the beginning of the air controllers' strike in August 1981.

Both Dulles International and Baltimore/Washington International airports have benefited to some extent from the airlines' failure to divide up the flights at National. Passenger traffic at both airports is running ahead of last year, and each continues to get new service, while flights and traffic at National lag behind last year's levels.

The number of flights at Dulles by major airlines and commuters was up 15 percent over last year through August, the last month for which data is available from the Federal Aviation Administration. More than 1.6 million passengers used Dulles in the first eight months of 1982, up 12.2 percent over the same period of last year.

Traffic at busier BWI has been growing at an even faster rate. The number of flights increased 19.5 percent in the first nine months, while passengers -- totaling more than 3.4 million -- were up 19.4 percent over the same period of 1981.

National is still the major airport in the area. In the first eight months of 1982, more than 8.6 million passengers used it. But the number of passengers was down 7.7 percent from the 9.4 million passengers in the comparable period of 1981. The total number of flights -- more than 152,000 -- was down 11.6 percent. ulles will get a real boost on Nov. 15 when People Express Airlines begins low-fare service from Dulles to Newark and two cities in Florida, marking the first time in years that Dulles will have daily nonstop jet flights to those points.

The fast-growing, no-frills airline will offer three daily flights to Newark, its major hub, and daily nonstop flights to both Sarasota and West Palm Beach at fares that are the lowest in the industry.

The new Dulles flights in part can be attributed to the failure of the airlines operating at National Airport to agree on how to distribute the 35 additional daily flights there. As a result of the carriers' impasse, the National landing rights -- or slots, as they are called -- were put back into the system by the FAA and made available only for flights at Dulles and BWI.

"We just can't satisfy all the demand from National, so we hope to spread some of it to BWI and Dulles," said People Express managing officer Harold J. Pareti.

The airlines have met repeatedly to distribute the National slots but failed. At the last meeting the dispute was so intense that nine airlines voted against a proposal that would have given each of them exactly the number of flights they wanted. They did so, they said, to keep New York Air and USAir from increasing the number of their flights.

Although many airlines want the FAA to step in and decide how to distribute the extra flights at National, Edward P. Faberman, FAA deputy chief counsel, suggested that one of the FAA's options "is to do nothing, to maintain the status quo."