The Thatcher government is moving toward a major restructuring of Britain's military forces during the 1980s that is expected to significantly reduce the size of the Royal Navy by phasing out about a third of its large warships.

The money-saving proposal by Defense Secretary John Nott is built around replacement of expensive hunter-killer submarines, smaller surface ships and versatile aircraft. This would change the way the British Navy fulfills its principal role in the North Atlantic Treaty Organization of protecting eastern Atlantic sea lanes from Soviet submarines in time of war.

Nott's plan reportedly also includes a reduction in the strength of the British Army, including the elimination of about 2,000 soldiers on headquarters staffs of the British Army of the Rhine in West Germany. This would reduce the number of British troops on the NATO front line in West Germany to the 55,000 required by treaty commitments.

Nott presented his plan yesterday at a closed-door meeting of Prime Minister Margaret Thatcher's Cabinet, which appeared to agree with its general aims, according to informed sources. But a decision on its final shape reportedly was delayed because of questions about the impact of the warships' cutback on employment in the shipbuilding industry here and on Britain's relations with the United States and other NATO allies.

The sources said consultations are to begin soon between Nott and U.S. and NATO officials on how the changes would affect NATO's naval defense of the Atlantic, which is shared by the United States and Britain. Critics of the proposed reduction in British naval strength have claimed here that it would weaken Atlantic defenses and shift more of the burden onto the U.S. Navy.

The restructuring plan, which emerged from a major review of long-term British defense spending, is designed to save Britain at least $10 billion during the coming decade. According to defense experts here, Britain needs the savings to meet its other military commitments, including a pledge to NATO to increase defense spending by an average of 3 percent a year above inflation.

A leading British defense analyst has estimated that Britain is trying to stretch a $12.3 billion annual budget to perform tasks and make improvements costing at least $15 billion annually. The strain comes, the analysts say, from the large number of roles the British military still tries to perform, the rapidly rising cost of new weapons technology, the expensive new Tornado fighter-bomber for the British Air Force and the modernization of Britain's independent nuclear deteerent.

The Thatcher government's controversial decision to replace Britain's aging Polaris submarine-based nuclear deteerent with U.S.-made Trident submarines and nuclear missiles is estimated by itself to cost nearly $12 billion during the next 15 years.

Nott's predecessor as defense secretary in the Thatcher government, Francis Pym, tried to economize with fuel, personnel and procurement cutbacks that hampered the military's day-to-day maneuvers and training. When Nott took office earlier this year, he decided to make a complete review to determine which military functions should be scaled down or could be performed more efficiently and cheaply.

With Trident, the British Air Force and the treaty commitment to keep 55,000 British troops in West Germany virtually untouchable, the Royal Navy was singled out for three-fourths of the desired savings, according to widely circulated reports here. One reported reason was that Nott became convinced that there was a more cost-effective way for the Navy to conduct antisubmarine warfare in the Atlantic.

Nott's plan reportedly relies on greater use of smaller, cheaper ships to do the job of technologically advanced British aircraft carriers costing nearly half a billion dollars each and quarter-billion-dollar frigates now earmarked as the "platforms" for sonar-carrying helicopters, fighter planes and missiles to hunt and destroy enemy subs. This appears to be the justification for phasing out or selling at least one new aircraft carrier and a number of destroyers and frigates.

In an intense debate in the Defense Ministry, military services, Parliament and the press, his critics have claimed, however, that the loss of these ships will make it impossible for Britain to provide adequate protection for naval convoys across the Atlantic in wartime and to join in NATO naval shows of force such as the current patrol of Western warships just outside the Persian Gulf.

Thatcher fired one of these critics, former Navy minister Keith Speed, who publicly called Nott's plan "rubbish" and warned that "irreversibly to run down the Royal Navy would be to ignore this country's history, its geography, its economic trading base and the security facts of life as members of NATO."

Nott has stressed in Parliament that the restructuring is not a cut in Britain's overall defense spending. He also is expected, according to informed sources, to tell U.S. and NATO officials that the changes would be accompanied by an extended British commitment to increase defense spending by the NATO target of 3 percent above inflation each year throughout the rest of the 1980s.

The Thatcher government will be adjusting this coming year's defense budget only for inflation, however, because last year's budget was overspent by 5 percent above the previous year after accounting for inflation. Even while standing still this coming year, Nott has said, Britain will have increased military outlays by 8 percent over three years under Thatcher, just below the NATO target.