Roaming beneath the world's oceans as an independent NATO deterrent force against the Soviet Union are the sleek lines of the British Resolution-class nuclear submarines, each armed with 16 Polaris missiles tipped with triple warheads.
Britain has two on patrol at any given time, invisible to probing Soviet detection systems. Two others undergo repairs and stand by.
Now, long-simmering controversy over the submarine force, and the complex political, economic, and military issues it embodies, has shot up through the waters of British public debate with new force.
The British Conservative government is the only political party in the country that still believes Britain must retain an independent nuclear deterrent and thereby preserve its diplomatic and prestigious seat at the big-power table.
But a new announcement from the Defense Ministry has intensified pressure on the government to change its priorities by merging British nuclear deterrence with overall NATO forces as a way of saving money.
Critics ask: Can Britain really afford an independent strike force anymore?
The US Defense Department has monetary and strategic interests in the debate here and is watching it closely.
The new announcement, made after news had leaked to the left-of-center Guardian newspaper in an apparent effort to weaken government policy, centered on the motors for the existing Polaris missiles.
It said Britain had asked the United States (under a previous agreement) to replace motors in all 64 missiles in use here, plus spares (which brings the missile total up to about 100). The department believes the cost would be ''several hundred million pounds,'' believed to be about L600 million ($1.1 billion) by defense correspondents.
This, it is said, would extend the life of the missiles into the next century. Defense officials say the move was signaled twice in 1980 and should cause no surprise.
But the news goes to the heart of one of Britain's biggest public policy debates of recent years - whether to replace the entire Polaris fleet with later-model US Trident subs and missiles.
Prime Minister Margaret Thatcher has decided to go ahead and buy four new Trident submarines and missiles from the US at a cost of L5 billion ($9.25 billion) by the early 1990s. Her argument was that Britain should continue to have its own independent defense, as befits a member of the nuclear club of nations, and that the Polaris fleet will be obsolete and vulnerable to Soviet detection soon after 1990.
Her government still has not decided which Trident system to buy. It it choosing between the existing US Trident C-4 missile and the bigger D-5. The size of the missile dictates the size and shape of the submarine hull.
But since her policy became clear, a number of events have taken place, and criticism has mushroomed.
The British recession has deepened. More than 3 million people are out of work. Industries are closing down every week. Government spending is being cut in nonmilitary fields - many a local council has complained that reduced grants meant they could not adequately clear snow in the recent record-breaking winter cold.
The cost of the Trident system has risen from a suggested L5 billion to an accepted L6 billion and to a widely projected 10 billion. This comes as Britain's nationalized industries require more and more public funds, North Sea oil revenue is somewhat less than expected as a global oil glut continues, and Mrs. Thatcher is under political pressure from her own party to ''reflate'' (spend more government money) to regain lost popularity as the 1983-84 elections approach.
The opposition Labour Party is committed to giving up British nuclear weapons altogether. The new Social Democrats would continue the Polaris, retain nuclear weapons, but give up Trident and merge nuclear forces into NATO's when Polaris is outdated.
The political reality now is that new rocket motors will extend the life of Polaris through the next century, thus strengthening Labour and the Social Democrats, as they say the exceptionally expensive Trident just isn't necessary any more. The government is also spending (STR)1 billion to replace existing Polaris warheads with the Chevaline, now being tested at Cape Canaveral in the US.
The suspicion grows among opposition parties that, in fact, the Thatcher government is now in a position to back away from its Trident commitment -- or at the very least, to delay it year after year to avoid committing money to it.
Speaking for political effect, but also summing up a common view these days, SDP defense spokesman David Owen (a former foreign minister) says the only ''sensible'' explanation of the decision to buy new Polaris rocket motors is that the Defense Ministry has already ''quietly decided to cancel Trident.''
The Defense Ministry position is that the Polaris fleet only costs about 1.5 percent of the 1981-82 defense budget (which is about 12.3 billion). It holds that the new Polaris motors would still keep overall strategic costs under 2 percent.
Critics say operating the Polaris subs costs another 2.2 percent of the budget a year. New Chevaline warheads will take another (STR)1 billion. Trident costs keep esclating all the time.
The Pentagon must now plan for the contingency that Mrs. Thatcher will delay Trident for two years, then be defeated in the next election. That would leave either a Labour or a Social Democratic-Liberal government committed against buying any Tridents at all from the US.
Many a Conservative supporter insists Trident is necessary. The alternative, they say, is entrusting the US with British defense. Critics say that is inevitable. Die-hard Tories say, ''never.''