London — The Royal Navy carrier HMS Invincible, with Queen Elizabeth II's second son, Prince Andrew, aboard, has been welcomed back to Britain from the South Atlantic amid scenes of wild jubilation.
But in the Admiralty in Whitehall a campaign as grim as that in the Falklands is under way to protect the ''senior service'' from what top sailors are describing as the government's plan to deprive it of teeth.
Among the ironies of the Falklands crisis, perhaps the greatest is that if it had broken out three or four months later, Britain might have been powerless to wrest the islands back from their Argentine invaders.
It was only by pressing into service such ships as the Invincible's aging partner, HMS Hermes, a number of older frigates, and several civilian cruise liners, including the QE2, that Margaret Thatcher was able to stitch together a credible force for the campaign.
Now the admirals are trying to convince her that the massive rundown in Britain's surface fleet, set to continue under existing policies, is a dangerous act of folly. They want the prime minister to order a halt in the Royal Navy's decline and lay new plans for the future, with the lessons of the Falklands crisis well to the fore in preparing the reassessment.
But Mrs. Thatcher finds herself in a bind. Her government is deeply committed to maintaining Britain's land and air forces at current strengths, and the plan to replace its Polaris submarines with the Trident submarines will account for a huge part of future Royal Navy spending.
Moreover, the government announced two years ago a comprehensive review of future defense policy, and heavy cuts in the surface fleet form part of the review. Mrs. Thatcher cannot give in to the naval lobby without agreeing to a wholesale revision of a strategy that her defense secretary, John Nott, continues to insist is correct.
The naval lobby, however, is highly influential, and the public debate following the Falklands campaign involves massive criticisms of government policy, with official leaks forming part of them.
It has been revealed, for example, that the First Sea Lord, Adm. Sir Henry Leach, warned Mrs. Thatcher nearly 18 months ago that planned cuts were unwise and irresponsible.
The influential naval manual, Janes Fighting Ships, in its latest edition gives current defense policies a pounding, arguing that the Falklands campaign revealed huge gaps in Britain's military capacity.
Janes points out that at the outbreak of the campaign the Royal Navy had only three ships with a modern defense system. Only 22 fighters were available for air cover. There was an almost total lack of airborne radar cover for the ships.
Battle conditions revealed that a number of frigates were liable to burst into uncontrolled flames when hit by enemy missiles. And the carrier Hermes was on the verge of retirement while the Invincible was about to be sold to the Australian Navy.
Much of Britain's naval planning in the past two decades or more has been based on the idea that any conflict would probably be with the Warsaw Pact forces. The Falklands campaign demonstrated that British naval power can be needed at short notice outside the NATO area.
It is on this point that the naval lobby, fueled by discreet support from within the Royal Navy itself, is beginning to focus.
Was the Falklands campaign a one-of-a-kind affair, or should Britain insure against a comparable crisis in future?
Mr. Nott, who has said he will not seek reelection at the next general election, argues that existing policies are right. His critics insist he is wrong.
Military observers say the debate is only just beginning and will intensify in the months ahead as the ''Falklands factor'' continues to impinge on official planning about the Navy's future.