Britain said today it was prepared to lift financial sanctions imposed on Argentina during the Falkland Islands war, an essential step toward rescheduling of Argentina's huge debt payment to foreign banks, including those in Britain.

A plan to lift the sanctions at midnight was postponed late tonight, however, because of "legal complications" in Buenos Aires, the British Treasury announced. It was not clear here whether the problems were entirely technical or also political.

Sources said they still expected the mutual ending of financial restrictions to take place soon.

In a separate action, the British government released a major study on the economic future of the Falklands and other small South Atlantic islands, which calls for investment of as much as $140 million to assure the region's long-term viability. The report calls for creation of a development authority, upgrading of the wool and fishing industries and a new airport.

Without at least some of these moves, the report by Lord Shackleton warns, "the internal economy of the Falklands is in grave danger of collapsing in the next five years."

The negotiations over the lifting of financial sanctions took place during last week's meeting of the International Monetary Fund in Toronto. Britain had been insisting that the sanctions be ended as part of an overall normalization of trade and economic relations, which Argentina refused to do. The British were finally persuaded to agree to the more limited step of unfreezing Argentine assets because of pressure from other IMF members to help cope with Argentina's $37 billion in foreign debts.

At the end of June, Argentina was already $2.3 billion in arrears on its interest bill this year of almost $13 billion. Without the unfreezing of about $1 billion of Argentine deposits in Britain and permission for British banks to join negotiations on rescheduling of debt payments, IMF representatives argued that Argentina's massive financial problems could not be solved.

All other economic restrictions imposed by the two countries when fighting broke out last April will remain in force, Britain's Treasury said today. This includes an arms embargo and restraints on regular trade.

Britain has unsuccessfully pressed Argentina to announce formally a cessation of hostilities so that relations between the countries could be restored. But Argentina has declined to make so categorical an acknowledgement of its defeat in the Falklands unless Britain agrees to some negotiation on the future of the islands. Prime Minister Margaret Thatcher has emphatically refused to do this.

As a result, Britain has made a number of unilateral moves such as returning all Argentine prisoners and reducing shipping exclusion zones in the South Atlantic. The financial arrangement, however, requires that both countries take action, which was only possible under the aegis of the IMF. Another factor was concern over the debt repayment problems of many countries, including Argentina which has one of the highest foreign debts in the world.

Even the readiness of both countries to unfreeze assets does not represent a formal agreement between the two countries, but was negotiated through third parties, British officials said. They said late tonight that because of their indirect contact with the Argentines they were unable to say how long the delay in implementation would be.

The continuing political uncertainty surrounding the Falklands is also one of the elements in the Shackleton report on the island's economy and future. The document is an updated version of a 1976 two-volume analysis of the South Georgia islands, undertaken by Shackleton, a 71-year-old former member of Parliament whose father is buried on South Georgia Island. Many of its conclusions are the same, but the passage of time and especially the war assure it greater attention.

The report's main contention is that substantial investment is required to revive an economy in serious trouble. Much of the land is now owned by absentee landlords. Shackleton recommends that a program begin to have the farms purchased by tenants to assure that a greater portion of the profits are recycled rather than returned to Britain. A decline in world wool prices is another reason for the economic difficulties on the islands and this too would be offset, the report asserts, by more owner-occupied farms.

To aid in this project and support extensive development of fishing, Shackleton urges creation of a Falklands development agency, a government-backed group. He recommends improvement of tourist facilities and communications and further exploration for oil.

Any significant oil search, however, he writes, is "unlikely until a stable political climate has been established in the region."

The cost of these projects would be about $60 million to $70 million.An even more costly and ambitious investment program would be required, Shackleton asserts, for development of the vast resources of krill, a high protein shrimp-like animal, in the South Georgia area. The cost of this, he estimates, would approach $80 million.

The figures, which the report acknowledges, are "quite substantial," are all broad estimates, but if the various proposals were successful the economy could expand by as much as 90 percent in the late 1980s. The alternative, Shackleton writes, is the "cost of supporting the islands over a long period of further decline" in population, output and potential.

The Thatcher government said in releasing the report that it will require "careful study".