The Civil Aeronautics Board said yesterday it has sent to the White House a recommendation that would result in a merger of Pan American World Airways and National Airlines into the nation's second largest airline.

The board itself formally approved the proposed merger, but the president must approve the transfer of National's intertnational routes to Pan Am.

Although National flies to five major European cities, the board decided not to allow Pan Am to acquire National's lucrative Miami-London route on grounds that it might lessen competition substantially anbd thus violate the antitrust laws. The board already has instituted a proceeding to decide what airline should succeed National on the route.

Under the law, the president can disapprove the board action only on the basis of foreign relations or national defense considerations, not on the basis of economic or carrier-selection considerations. If he fails to act within 60 days of getting the board's decision, which was Oct. 25, it becomes final.

The board's action brought Pan Am a giant step closer to attaining its goal of acquiring a network of domestic routes to supplement and feed its extensive international network. It also acquires a fleet of 55 airplanes at a fraction of their cost in today's market.

The combined Pan Am-National will be second in size only to United Airlines.

Although the board also formally approved Texas International Airline's proposed acquisition of National, Texas International already dropped out of the scramble for National when it agreed in July to sell to Pan Am for $50 a share the 2.1 million shares of National it had acquired

Last month, the CAB turned down the bid of another suitor, Eastern Airlines, to acquire control of National on the grounds that a merger between the two Miami-based airlines would lessen competition substantially.

So far, Pan Am has acquired 61.8 percent of National's stock and has options from Texas International that will bring its total to 77.1 percent.