Lord Carrington, Britain's foreign secretary, will fly to Moscow next week to negotiate on the Afghan peace conference proposed today by the European Economic Community.

The prompt diplomatic follow-up underscores the European leaders' desire to broaden the Community's involvement in global diplomacy.

Plans for Carrington's trip on July 5 and 6 were announced simultaneously in Moscow and in Luxembourg, where Common Market leaders adopted the British-drafted peace plan at the conclusion of a two-day summit meeting.

The Afghan initiative avoids the main pitfall of previous plans by leaving the tricky question of who represents Afghanistan until a second stage of the process.

The British plan is a refinement of an earlier peace call by Valery Giscard d'Estaing, then president of France. But Britain has added the idea of two stages to the conference and also carefully prepared the ground by sounding out all the proposed participants -- except Afghanistan -- before announcing the plan.

The plan proposes that nine nations directly concerned with the Afghan problem meet in the fall to attempt to gain Afghanistan's independence as a nonaligned state under guaranteed security safeguards. In its second stage, which would take up Afghanistan's future status and how to implement the guarantees.

[In Paris, Afghan Embassy Counselor Hamid Nezam said the Kabul government rejects Western proposals for an international conference but is prepared to meet Pakistan and Iran, its neighbors, to settle outstanding problems. At a press conference, Nezam said his country refuses "to turn Afghanistan into an international issue," Reuter reported.]

While the chances for success in his mission remain unclear, Carrington has told other Western leaders that he believes Moscow may be ready to seek a face-saving way to start extricating itself from Afghanistan, where it intervened under the terms of a friendship treaty on the gounds that the Kabul government was threatened by outsiders.

Ultimately, the European plan could open the way to a major reduction in East-West tensions if an Afghan formula can be found based on a nonaligned state guaranteed by its neighbors and the big powers against any outside interference.

The Reagan administration, which apparently feels unready to extend negotiating feelers to Moscow, has encouraged the European move. But U.S. officials can be expected to be on guard against the possibility that Moscow will use Afghan talks as a way to reduce international criticism of Soviet behavior there when the West remains concerned about a possible Soviet intervention in Poland.

[In Washington, Secretary of State Alexander M. Haig Jr. issued a statement citing the "profound importance" of the European initiative.]

European summit participants avoided any public mention of Poland, perhaps because they are anxious to get Moscow to start negotiating about Afghanistan. Technically, the Soviet Union, which dislikes dealing with the Common Market as a political entity, is treating the proposals as a British initiative, not a European one. But Soviet accceptance would amount to a new recongnition of European diplomacy.

An Afghan success would be particularly timely for the Common Market because its long discussed Middle East initiative is floundering. To the disappointment of Carrington, who had hoped to make dramatic strides toward an Arab-Israeli settlement during the next six months when Britain is leading the EEC, the scope for any European action is hard to see.

Although Carrington isists that Europe will continue seeking a way to implement its Venice declaration calling for mutual concessions between Israel and the Palestine Liberation Organization, the Common Market appears to be in a quandary about what practical steps to take.

French President Francois Mitterrand, attending his first international meeting since the Soicalists took power, said today that he had reservations about the approach pursued by Europe so far.

On the economic front, Community leaders admitted today that they lacked a common approach on how to remedy European economic problems or what to say the President Reagan next month at the Ottawa summit of industrialized countries.

Most of the discussion at the summit here was spent trying to reach a consensus on Europes's economic predicament. But leaders emerged with an agreement to continue disagreeing about whether the time has come to shift from inflation-fighting to stimulating employment.

Mitterrand urged Common Market countries to adopt similar policies, such as reducing the work week, to start an early economic recovery and improve employment.

The French leader said that he was convinced of the need for a European dimension and European coordination to improve social conditions. But Great Britain, West Germany, Italy and Belgium insisted that inflation first needed to be controlled by applying austerity policies. However, Chairman Paul Simon (D-Ill.) said the Soviets did not express any views on the visit here next week of the British foreign secretary, Lord Carrington, who will propose two-tiered negotiations toward a settlement on behalf of the European Community.