In a move that enables the Carter administration to avoid a sensitive foreign policy decision, Mexico has dropped its bid to buy 26 U.S. F5 jet fighters worth about $150 million.

Mexico's request, made late last year, had triggered a brisk, behind-the scenes debate within the U.S. national security community because it involved two conflicting policy goals.

Selling the planes would have run counter to President Carter's policy of setting an example for other armsproducing nations by reducing U.S. foreign military sales, particularly in Third World areas.

But, rejecting the request would have offended the Mexican government and jeopardized the special relationship Carter has sought to establish with President Jose Lopez Portillo. Carter wants increased Mexican cooperation to help combat drug traffic and the flow of illegal alliens.

he Reliable sources said the Mexican government backed away from the plane purchase because press disclosure of the request had stirred opposition within Mexico. Critics there hava charged that Mexico, which is struggling with severe internal economic problems, cannot afford the planes.

The sources added that withdrawal of the Mexican request makes it almost certain the Washington will turn down a separate bid by Guatemala to purchase six less sophisticated F5s worth about $30 million.

The administration had been unenthusiastic about the Guatemalan bid because of the country's poor human rights record and its threats to use force in its border dispute with Belize. However, administration sources had noted, it would have been difficult to turn down Guatemala if F5s were sold to neighboring Mexico.

The F5, manufactured by the Northrop Corp., is a lightweight, easily flown and simply maintained fighter tailored to the needs of the world's smaller air forces. The United States has supplied it to a number of Third Wo countries, particularly in the Middle East and, more recently, in Latin America.

Mexico's air force is equipped with about 100 outmoded subsonic jets, many of them more than 20 years old, and is regarded in military circles as badly in need of refurbishing with newer equipment.

For that reason, some administration officials, noting that Mexico has shown great restraint in its military spending had argued in favor of the F5 sale.

Others countered that Mexico, which faces no external threat, does not need a relatively sophisticated supersonic plane like the F5. They had suggested offering Mexico a less expensive, subsonic type of military jet.

The sources said Mexico, in notifying Washington that it was withdrawing its bid not indicate what it will do now. The possibilities, according to the sources, are that it will ask for subsonic planes or turn to another country such as France for supersonicfighters.