The Reagan administration will be rebuffed if it tries to make Britain sit down at a negotiating table with Argentina over the Falkland Islands.
At least, that is the situation at the moment, in the flush of post-fighting victory here in London. Private comments from official after official close to Cabinet ministers make it clear it will be the situation as long as Margaret Thatcher remains in charge.
Officials are well aware that the United States wants Britain to negotiate a rapprochement with Buenos Aires and a long-term solution to the Falklands. President Reagan made the point to Mrs. Thatcher when they met in Washington June 23.
''But you Americans don't fully realize, I think,'' commented one senior man, ''that our hearts are hardened against giving up what is ours.''
''We haven't gone through what we've just gone through to hand the islands over to someone else,'' another said sharply.
''The moment we took casualties and realized we'd have to invade the Falklands to retake them,'' another said, ''all previous bets were off.
''We were prepared to offer concessions in April and early May, that is true. But Argentina rejected them all. We looked at seven different sets of proposals from your secretary of state, from Peru, from the United Nations, and others. We tried.''
But Britain wants to avoid the impression that the Falklands are still a ''colony,'' and will push for some form of self-government.
When asked by Scandinavian television June 21 if she would negotiate with Argentina, Mrs. Thatcher replied tartly: ''Certainly not. Why should I? . . . Is anyone suggesting that I should hand over our people to a regime that doesn't know human rights in the same way as our people do? No.''
The question now is whether Mrs. Thatcher's attitude, which springs from deeply held conviction as well as a shrewd reading of the mood among her government's backbenchers in the House of Commons, can withstand US, West European, and UN General Assembly pressure in the months ahead.
Some in the US suspect she is merely staking out a bargaining position with the US and Argentina. By appearing inflexible at the start, the theory goes, she hopes to extract concessions later - such as US troops in a multinational peace-keeping force on the tiny, remote, underpopulated islands.
Reagan administration sources are quoted here as saying a long-term deal with Argentina is inevitable, given the closeness of the islands to the Argentine mainland.
Argentina might simply refuse to declare hostilities at an end and force Britain to keep a larger garrison force on the islands than it wants to or can really afford.
US and West European officials say the islands are too far away and too small to be able to stand on their own feet economically or politically, especially if Argentina remains hostile.
Unless Mrs. Thatcher shows more flexibility, the US thinking goes, the real loser in Latin America could be the US for going to Britain's aid in the first place.
Mrs. Thatcher does not see it that way, however, and she has authorized Foreign Secretary Francis Pym and Defense Secretary John Nott to make government policy bluntly plain.
She also knows that Conservative Party hard-liners, led by Northern Ireland Unionists such as Enoch Powell, would raise politically damaging opposition if she backed down now. To Mr. Powell, the war is a way to warn the government not to weaken British ties with another Protestant enclave - in Ulster.
The Falkland Islands are British, officials insist in separate interviews. They will remain so. To allow any Argentine role in their sovereignty would ''dishonor'' the 250 or so men who were killed in the 74-day war, and the entire war effort.
This view is put to foreign newsmen with the intense emotions the war unleashed here. To many middle-aged (and some younger) officials, Britain has rediscovered a measure of national purpose in the war they believe was fought for principle and justice.
Argentina is spoken of disparagingly. Its troops' actions in preparing to use napalm, the way residents of Goose Green were locked into a hall for days on end , the way Argentine forces opened fire on British troops after first showing a white flag to lure them on, and the filthy state in which Argentine soldiers left many Port Stanley houses have led to official icy disdain.
Argentina is to be cut out of any talks on the islands' future, except those in which Argentina might play an economic role.
A garrison of at least 2,000 British forces will repair and lengthen the Port Stanley airstrip, build new roads, and put up its own permanent quarters.
Mrs. Thatcher will guard against future Argentine attacks with her own forces , planes, ships, and submarines. But she will keep on pressing President Reagan for US troops in a multinational peace-keeping force. Mr. Reagan is likely to say no.
Already a military administrator, Maj. Gen. Jeremy Moore, has been appointed. To avoid the ''colony'' image, former Gov. Rex Hunt has been renamed ''civilian administrator.'' In a few months, the 1,800 islanders will be asked what form of administration they themselves prefer.
At the moment, officials in London say Mrs. Thatcher sees no future in complete independence, but that ''self-government'' is an ''infinitely flexible term.''
The Cabinet has in mind a gradually increasing amount of local autonomy, leading perhaps to the status of an ''associated state,'' with Britain responsible for foreign affairs and defense.
This was the status of St. Kitts-Nevis, Dominica, Grenada, and Antigua-Barbuda in the Caribbean before those former British territories moved to complete independence.
Conservative backbenchers here thunder that the Falkland Islanders will want to stay British forever now that they have felt the callous hand of Argentine occupation.
Mrs. Thatcher has ruled out UN trusteeship, on the grounds that it would allow Argentina partial access to island government.
Whether Britain or a multinational force keeps the peace, the prime minister wants to create an air of stability. She hopes many immigrants will swell the islands' population (already some 700 applications have arrived in London from Britain and the Commonwealth) and develop small-scale farming, krill (shellfish) resources, wildlife reserves, tourism, and possibly other industries. Eventually , London hopes major oil companies will be encouraged to drill for oil and natural gas on the continental shelf around the Falklands. A 1970 study by the Shell Oil Company said the area could be rich in oil.