In a major victory for British efforts to put diplomatic and economic pressure on Argentina to withdraw its forces from the Falkland Islands, the other nine European Economic Community countries decided today to join Britain in banning all imports from Argentina.

The ban is the first such blanket sanction imposed by the Common Market in its quarter century of existence. It barred only selective exports to Iran when Americans were being held hostage there and to the Soviet Union after the invasion of Afghanistan. The European Community has restricted only a small fraction of imports from the Soviet Union in response to the military crackdown in Poland.

Implementation of today's decision in Brussels by ambassadors from all 10 member countries--Britain, West Germany, France, Italy, Belgium, the Netherlands, Luxembourg, Denmark, Ireland and Greece--is expected Wednesday. The ambassadors had prohibited arms sales and deliveries to Argentina yesterday.

Britain failed only to win approval of a suspension by all European Community countries of export credits to Argentina. The ambassadors decided to leave this up to individual member countries, some of whom have already indicated they will extend no new credit to Argentina during the Falklands crisis.

A communique issued by the ambassadors condemned the Argentine invasion and called on other countries to support the Europeans' efforts to achieve a peaceful withdrawal of the Argentine forces. But Common Market diplomats disagreed over whether this meant the Europeans were formally requesting other nations to join in these sanctions against Argentina.

Australia has already banned imports from Argentina and British diplomats said other Commonwealth countries such as Canada may act now that the Common Market has acted. The United States has not taken any such steps while trying to mediate a diplomatic settlement of the crisis.

"We are very, very pleased with this response," a British Foreign Office spokesman said. "It could hardly have offered more."

Today's action could affect more than one-fourth of Argentina's total exports. In 1980, Common Market countries bought nearly $1.7 billion worth of Argentine goods, including meat, animal feed, leather, wool, oil products and steel.

Argentina condemned the decision today and threatened to retaliate by refusing to buy European products, now limited to non-military goods. A statement by the Argentine Foreign Ministry said the import ban was "a violation of international law which sets a dangerous precedent for the future of international economic relations, particularly those between developing and industrialized countries."

The Argentine statement said, "This measure sets a substantial example of what the developing countries can expect when they clash with the interests of Common Market members in defense of their own legitimate national interests."

Britain had been pressing for the Common Market action for days. Common Market diplomats suggested some countries overcame initial reservations about the unprecedented step in the hope it would help increase international pressure on Argentina sufficiently to avoid military action by Britain.

Prime Minister Margaret Thatcher held a luncheon meeting at her country home, Chequers, with Foreign Secretary Francis Pym and Defense Secretary John Nott to discuss the Falklands crisis. They have stayed in touch with Secretary of State Alexander M. Haig Jr. since he left London for Buenos Aires on his mission to mediate a peaceful settlement.

The Thatcher government also has promised to consider a letter signed by 15 Falkland Island civil servants--including the police chief, school superintendant and chief medical officer--urging that "a protecting power be appointed to help to arrange the temporary evacuation of the civilian population of these islands, under the terms of the Geneva Convention."

Addressed to the British foreign secretary, it was taken to the British Embassy in Uruguay by someone who had left the Falklands. A Foreign Office spokesman said the International Committee of the Red Cross has been asked to "interest themselves" in the well-being of the Falklands' 1,800 residents.

In addition, a British businessman with a half interest in a Falklands sheep farm, Edmund Carlisle, brother of a Conservative member of Parliament, told British reporters in Argentina after arriving there from the Falklands yesterday that many islanders are ready to forgo British rule.

He was reported here as saying they were afraid of the consequences of British military action against the Argentine occupation force and that the Argentines have promised improvements in their lifestyle, including local television, radio and newspapers.

This news caused a stir and a flurry of claims and counterclaims here today. Thatcher has insisted that any negotiated settlement of the sovereignty dispute following an Argentine withdrawal would have to be acceptable to the Falkland islanders, who have always opposed any compromise of British sovereignty in the past.

The deposed British colonial governor of the Falklands, Rex Hunt, and former islanders at the Falkland Islands Office here told reporters that both the civil servants' letter and Carlisle's claims were unrepresentative of what they believed to be the attitude of most residents of the Falklands. They pointed out that none of the elected members of the local assembly nor the deputy governor signed the letter to Pym.