BRITISH Prime Minister John Major flies from the United States to Moscow this weekend to give Presidents Mikhail Gorbachev and Boris Yeltsin details of an emergency food-aid package aimed at heading off widespread famine in the Soviet Union this winter. He will be the first Western leader to visit the Soviet capital since the failed coup.Mr. Major will be acting as chairman of the Group of Seven (G-7) nations and on behalf of the 12-member European Community, as well as his own government, British officials say. He discussed the emergency measures with President Bush when the two leaders met at the US president's retreat in Kennebunkport, Maine. The prime minister's unscheduled Moscow visit, Downing Street officials say, is part of a broader British initiative to orchestrate international short- and longer-term economic assistance in response to the Soviet crisis. "I look forward to discussing the way ahead with President Gorbachev and President Yeltsin and other leaders of the reform movement when I visit Moscow on Sunday," Major said as he arrived in Washington en route to Maine. "It is probable that we will actually have to provide food to the Soviet Union, rather than just food credits. I will not let the Soviet people starve." Major said developments since the coup presented "a historic opportunity for the people of the Soviet Union" and had "opened the door to the development of democracy, freedom, and prosperity." A senior British official says Major's initiative as current G-7 chairman was being taken at a number of levels. The official explains: "The most urgent problem is making sure that enough food gets to the Soviet people before the cold season arrives. But we have to demonstrate methods of preventing food and milk going bad, and making sure that other foodstuffs reach their destination in good condition." Beyond that, there would be a need for an enhanced and carefully planned program of technical assistance to meet the longer-term needs of the Soviet people, the official says. Aides say Major shares Bush's reservations about supplying the Soviet central authorities with large-scale cash aid in line with German and French demands. But Major favors dealing directly with individual Soviet republics and municipal authorities. Some of the problems created by the rapid disintegration of the Soviet Union were high on the agenda of EC foreign ministers at their meeting in Brussels on Tuesday. Afterward British sources said convoys of trucks were being assembled in EC countries, and being placed on standby to take food to the Soviet Union. The sources say huge stocks of beef and butter accumulated by the EC under its common agriculture policy could be released. In common with other EC countries, Britain has a strategic food stockpile originally intended for use in a defense emergency. "We have a rough target date of early October," a British source says. "That is when temperatures begin falling sharply there, and we want to be ready with the necessary food." British officials made it clear that the prime minister would be pressing Bush to throw US support behind an enhanced technical aid program for the Soviet Union. The technical aid package discussed by G-7 leaders at their summit meeting in London last month is now regarded by the British government as inadequate to meet the new situation in the Soviet Union. It was reported yesterday that Major will urge the London-based European Bank for Reconstruction and Development to raise its ceiling on loans to Moscow. This is seen in British government circles as a prelude to greater direct support from larger international institutions such as the International Monetary Fund and the World Bank. Major is said by his officials to be close to supporting full Soviet membership in the IMF.