Britain's commitment to the Falkland Islands will require a substantial increase in the country's defense budget, amounting to more than $1.6 billion for new equipment over a period of several years and almost $1 billion to maintain the British garrison next year alone, Defense Secretary John Nott said today.

Releasing a government statement called "The Falklands Campaign: The Lessons," Nott said the costs of securing the islands would be added to the government's existing spending plans, which meet NATO targets for a 3 percent annual increase over inflation. Nott said total defense spending for next year will be about $25 billion.

What the Falklands figures mean, military analysts said, is that the annual increase in defense spending between now and 1986 will be about 6 percent a year or double what was anticipated before the war last spring.

For a country in the grip of an intractable recession and with a government seeking to restrain the growth of public spending, the billions in extra costs represent a substantial financial burden.

In the aftermath of the Falklands, Nott said, "We shall be taking measures which will strengthen our general defense capability by increasing the flexibility, mobility and readiness of all three services for operations in support of NATO and elsewhere."

The $1.6 billion will replace all equipment lost in the conflict. It will also buy such Falklands-related equipment as long-range air refueling tankers that can double as troop carriers, Phantom and Harrier jet fighters, Chinook helicopters and antimissile weaponry.

The garrison costs for next year will cover the upgrading of existing facilities and building of new ones, such as airstrips and permanent barracks. In addition, the government last week announced a $50 million multi-year program for civil improvements on the islands.

The total cost to Britain of the Falklands affair -- from recapturing the islands to defending them for at least another four years and making them more habitable for their 18,000 residents -- will be about $4.8 billion, according to a report in yesterday's Guardian. Today's Ministry of Defense study supports that estimate.

Nott's report underscored his previous assertions that the main military threat to Britain is posed by the Soviet Union and that NATO, therefore, "must still have the first call on our resources." However, the report added, the Falklands war demonstrated Britain's need to be ready for "out of area" activities that require a degree of miltary flexibility that had not been fully accounted for in earlier defense plans.

There had been speculation that Nott would reverse last year's announced cuts in the size of the surface Navy because of the role played by the ships in the war. While Nott did say four modern frigates will be ordered to replace ships lost in the fighting and four other ships due to be mothballed will remain in the fleet, he indicated that the basic strategy of cutting the surface fleet will be maintained.

Britain's plans to purchase the Trident nuclear submarine system from the United States to provide the basis of its nuclear deterrent in the years ahead -- reaffirmed again immediately after the Falklands conflict ended -- has been criticized by the opposition Labor Party and it was again today in comments on the Falkland report.

Labor defense spokesman John Silkin, speaking in Parliament, said the government was "endangering its whole maritime policy" by insisting on buying Trident.

As a study of how the Falklands war conflict was waged and why it was successful, today's document lavishes praise on the country's professional military and the civilian support, especially the merchant marine, that was involved in the war.

"The campaign confirmed that the British people and their government have the will" to defeat an enemy half a world away, it said.

Specifically, the study says that the Falklands conflict shows that larger stocks than expected of ammunition and materiel were necessary, that civilian ships can be converted quickly to provide military support, that helicopters are in constant use on the modern battlefield and finally, that antimissile defenses have to be improved.

The war provided Britain's first experience of battle in the missile age, the document observes, and required Britain to undertake its "first large-scale amphibious operation for many years." All these experiences and others should prove a benefit to Britain's role as a NATO partner, the document concludes, and "the deterrent posture of the NATO alliance as a whole has been strengthened."