London — Britain's newly returned Conservative government is taking vigorous steps to place the defense of the Falkland Islands on a stable, long-term footing. A large twin-runway airport is to be built at a cost of more than (STR)200 million ($305 million). Construction will begin in the fall. After 1985, wide-bodied jets will be able to fly in and out of the remote South Atlantic dependency, reinforcing the British garrison there.
Plans for the new airport for ''Fortress Falklands,'' which will be on a different site from the present airstrip near the capital of Port Stanley, were announced in the House of Commons by Defense Secretary Michael Heseltine. He was immediately assailed by Labour for the ''ludicrous cost'' of the project.
The government's parliamentary foes also complained of a plan to use South Africa as a staging post for the British workmen who will be employed in building the new airport and roads associated with it.
The opposition says using South Africa in this way will cut into United Nations' resolutions which seek to prevent South Africa from being drawn into defense arrangements.
But Heseltine enjoys the backing of Prime Minister Margaret Thatcher, who promised during the election campaign to go on defending the 1,800 people of the Falklands. Mrs. Thatcher sees the new airport, to be established at Mount Pleasant, between Port Stanley and Darwin, as essential in the future supply and defense of the Falkland Islands.
The decision to build the airport confounds commentators who said during the election campaign that the government might reverse its commitment to the Falkland Islands and seek a diplomatic solution to their future. Now it seems that Mrs. Thatcher is not looking for talks with Argentina, even though the United States sees a political settlement as the only viable long-term option.
Mr. Heseltine said the establishment of an airport would enable Britain to keep a smaller garrison on the islands, as there would be the possibility of ''turning round'' military units sent there on duty. At present, units do stints of several months at a time in the often severe conditions in the South Atlantic.
Royal Air Force aircraft travel to the Falklands via Ascension Island, using complex and risky midair refueling. With the new airport, long-range aircraft will be used.
The new leader of the opposition Social Democrats, Dr. David Owen, has said establishment of a larger airport is necessary, given the government's commitment to go on defending the islands. But he urged Mrs. Thatcher not to let the airport become a symbol of continuing British sovereignty.
''Negotiations should be opened with Argentina, with the aim of a settlement within the lifetime of the new Parliament,'' he said.
But government ministers think it most unlikely that Mrs. Thatcher will be interested in negotiations for some time to come.