President Carter yesterday approved a major expansion of U.S. air service to Europe, authorizing nonstop service form 11 new American cities and letting three airlines fly across the Atlantic for the first time.

In a decision certain to be controversial, however, the President rejected the Civil Aeronautics Board's proposal to give Pan American World Airways a new Dallas-London route and said he would give the route to Braniff Airways instead.

Mary Schuman, assistant director of the White House Domestic Council who handles the President's aviation matters, yesterday denied suggestions that politics - namely, the intense interest and lobbying of Texas politicians and congressmen - had anything to do with the President's decision granting Braniff the new route.

"The President made his decision on foreign policy grounds," she insisted. "He has a public policy of favoring aggressive competition in international aviation." Part of that policy means eslecting aggressive, competitive carriers that will develop new makets and fares vigorously.

She noted that the President's decision to go with Braniff was consistent with the rest of the DAB's overall plan to give new European routes to strong regional carries: Delta Air Lines, Northwest Airlines, and National Airlines.

Pan Am didn't view the decision the same way. In a stinging statement issued shortly after the President's decision was made public, William T. Seawell, chairman fo the board and chief executive officer, said Pan Am was "outaged" and "fed up with government action which continues to weaked Pan AM."

"we do not believe that the President's decision was governed by considerations of foreign policy," Seawell charged. "Rather, it appears to have been dictated by the kind of political manipulation that the President promised would not characterize his administration."

Texas Gov. Dolph Briscoe visited the White House recently, and the Braniff award also was given the strong support of 20 congressmen. Some observers of aviation policy have hinted that foreign trade negotiator Robert S. Strauss, a former Braniff director, also may have been infulential in Carter's decision.

Schuman yesterday categorically denied such speculation. "I can state unequivocally that Robert Strauss had nothing to do with it, was not consulted and, as far as I know, ded not confer with the President about it," she said.

The new Atlanta-London and Dallas-London routes are especially desirable form the ailines' point of view. Under the terms of the agreement signed this summer with the British, American carriers exclusively may fly those two routes for a three-year peroid, after which a British carrier may also fly those routes.

(Under the same agreement, a British carrier won an exclusive London-Houston route for three years, with an American carrier to be added after that. Pan Am was named the airline to get that route.)

In simplest terms, Braniff had argued beforr the CAB that it should get the new Dallas route because its extensive feed system in the Southwest could serve the public Texas-boarding passengers better.

The board's decision had favored Pan Am in a 4-1 vote, with Elizabeth Bailey voting for Braniff.

Less controversial provistons of the President's decision include:

Atlanta: Cleveland: Denver; Houston: Kannsas City, Mo.; Minneapolis/St. Paul: New Orleans; Pittsburgh; St. Louis; and Tampa are opened up as "gateway" cities to Europe, in addition to Dallas.

Delta Air Lines was granted authority to serve the Atlanta-London route.

Nation Airlines was awarded routes between Miami, New Orleans and Tampa to Paris.

Northwest Airlines gets various routes between U.S. cities (Seatle: Portland: Los Angeles; Mimeapolis/St. Paul; Chicago: Detroit. Washington/Baltimore: New York/Newark; and Doston: and Glasgow and points in Dennask. Norway, Sweden. Finland and Jceland.

Trans World Airlines' existing authority is renewed, and it additionally is authority is authorized to serve Pittsburgh. Denver. St. Louis. Clevelan, Minneapolis/St. Paul, and Kansas City direct to Europe.

Most of Pan Am's existing autherity is removed and nonstop Miami-Medred authority is added, in addition to the Houston-London authority it is able to stand in three years.