London — On Thursday, May 27, an ''exclusion zone'' covering some 1 million square miles of South Atlantic waters will be imposed. The zone includes the entire coast of Argentina, but it doesn't originate from the warring nations of Britain and Argentina. Rather, it is from Lloyd's of London, the giant British insurer.
Any commercial ship venturing into that vast area after then will either have to carry new war-risk insurance or sail without coverage.
Since the Falklands hostilities began, war-risk insurance premiums have doubled. Because the 400-odd underwriters represented at Lloyd's directly or indirectly insure perhaps 90 percent of the world's merchant shipping, the exclusion directive issued from its Lime Street headquarters here has broad implications.
Curiously, Lloyd's underwriters continue to write coverage for Argentine ships. However, under orders from the British government, Lloyd's has been instructed not to pay any claims should they arise from Argentine shippers, but instead to put the funds into an escrow account.
''There's no question at all about Lloyd's defaulting,'' says a spokesman. ''There will just be a postponement of payment until conditions return to normal.''
As many as 60 merchant ships are currently believed to be holed up in the port of Montevideo, Uruguay, unwilling to sail to Argentina to take on grain or deliver their cargoes.
It is expected that at least some of these ships might be diverted to the United States to take on American grain if Argentine sources remain closed. Such a move could have an adverse effect on wheat shipments from Argentina, which is a major wheat supplier to the Soviet Union.
Not affected by the new Lloyd's exclusion zone are perhaps 50 civilian British ships pressed into military service to become part of the invasion task force. All insurance on these vessels is being carried by the British government.
In addition to being compensated for the use of their vessels, some ship owners will be paid for lost business as well. This is of particular importance to the two cruise liner operators, Cunard and Pando. Their liners are now doubling as troop ships or, in one case, as a hospital ship.
Pando officials decline to disclose the amount of payment the firm is receiving for its 45,000-ton cruise liner Canberra, which apparently carried assault troops over the weekend to the East Falkland Island beachhead. The line does say, however, that it has been assured by the British government that it will be ''no worse off financially as a result of the requisition.''
This is taken to mean Pando will be compensated for the loss of profits it would have realized had the Canberra and Pando's 14,000-ton liner, the Uganda, made their normal pleasure cruises to the Mediterranean and Scandinavian waters this vacation season.
Cunard, which owns the 66,850-ton Queen Elizabeth 2 (QE2) luxury liner, now also on military duty, is assumed to be operating under similar financial conditions.
It has been reported in London that Cunard had anticipated that the QE2 might be requisitioned well in advance of the actual call-up. As a result, it reportedly took out from Lloyd's a $10.8 million policy to cover loss of profits. Premium on that deductable policy was said to be $675,000. Cunard declines to comment on such coverage.
Subject of ongoing discussions here is what price should be put on the merchant ships now part of the South Atlantic task force if they were to be lost. The replacement cost of a ship would naturally be far greater than either original cost of construction or its current depreciated value.