France and Mexico dealt a major diplomatic blow to the U.S.-backed government in El Salvador today by officially recognizing leftist Salvadoran guerrillas and their political allies as "a representative political force" legitimately entitled to negotiate with the current government.

The joint French-Mexican declaration, submitted in New York for distribution to members of the United Nations Security Council, could result in a new diplomatic confrontation between the United States and Mexico and strain U.S. relations with the previously diplomatically reserved Socialist government in Paris.

The immediate effect of the carefully phrased declaration is to lend vital international support to the guerrilla alliance's attempts to pressure the Salvadoran government to accept a negotiated solution to the civil war, which has cost more than 22,000 lives since January 1980. It may also be a first step toward diplomatic recognition of the guerrilla alliance as an alternative to the military-civilian coalition supported by Washington.

In Washington tonight, the State Department issued a statement that reiterated the U.S. policy of rejecting negotiations with the guerrillas in favor of national elections in El Salvador next March. The statement appeared designed, however, to avoid the appearance of diplomatic conflict on the El Salvador issue.

"The (French-Mexican) declaration makes a number of important points with which we agree," the State Department press response said, pointing to the communique's reference to "authentically free elections." The response said administration officials "remain committed to the development of the electoral process."

The text of the France-Mexico communique was settled on after weeks of consultations between the two governments, including direct talks between Mexican Foreign Minister Jorge Castaneda and French Foreign Minister Claude Cheysson, who traveled through Central America this month. Both ministers had expressed concern that El Salvador's conflict will spread throughout the Central American region.

The preface to the text released today, while noting that U.N. Security Council President Jorge Illueca of Panama has been asked to distribute the communique to council members, does not ask for any specific action by the United Nations.

Normally, full diplomatic support of El Salvador's guerrilla movement by foreign governments would begin with recognition of a "state of belligerency" in the country, but such a move usually depends on insurgents occupying clearly defined geographical areas and maintaining an organized governmental structure. The guerrillas and their allies have fulfilled neither of these conditions.

A Mexican Foreign Ministry source said today's declaration comes as close as possible to recognizing a state of belligerency without actually doing so.

Both Mexico and France still have diplomatic relations with El Salvador's current regime.

Washington is currently supplying more than $30 million of military aid and $136 million in economic aid to bolster the regime, which is composed of military officers who came to power in an October 1979 coup, and Christian Democratic politicians. This week Washington sent four more helicopters to the Salvadoran Army to supplement ten leased to the regime in January. There are 46 U.S. military trainers in El Salvador giving the Army technical and strategic advice.

Although the U.S.-backed government has initiated major economic reforms, fighting with leftist guerrillas has increased and the regime's soldiers are frequently accused of civilian massacres and torture.

Washington maintains that the insurgents are receiving advice and substantial amounts of arms from Cuba and other Soviet-allied countries, which U.S. officials say would dominate any revolutionary regime if the insurgents win by force and subvert any negotiated "popular front" government that included the guerrillas.

For more than a year, however, the largely Marxist guerrillas have sustained an alliance with moderate Social Democratic politicians and have lobbied extensively for international support, especially among European socialists and in Mexico, which frequently has clashed with Washington over U.S. policy in Central America. Mexico maintains that revolution in the area is inevitable and need not automatically lead to alignment with the Soviet Bloc.

The United States and the current Salvadoran government have rejected initiatives to negotiate the structure of the regime itself. Instead, they have repeated that the leftists are free to participate in the upcoming elections for a national assembly.

The guerrillas say such elections are no solution because of the physical danger to leftist candidates in El Salvador, the potential for government fraud, and because the Army would still hold effective veto power over the results.

In the communique today, France and Mexico "recognize that the alliance of the Farabundo Marti National Liberation Front a collection of four armed guerrilla commands and the Democratic Revolutionary Front composed of the guerrilla's mass organizations and leftist political parties constitute a representative political force prepared to assume the obligations and exercise the rights they derive from that status . In consequence it is legitimate that the alliance participate in the restoration of mechanisms of rapprochement in negotiations necessary for a political solution of the crisis."

While saying the search for a solution should be left entirely to Salvadorans, the communique endorses "fundamental changes" to establish a "new internal order" in El Salvador advocated by the left but only partly supported by the current regime. This new order would include a restructuring of the armed forces, which now wield most of the power in the government, as well as "creation of the necessary conditions for ...authentically free elections."