LONDON, FEB. 23 -- Former U.S. defense secretary Caspar Weinberger today joined the rarefied ranks of 55 Americans, including Dwight D. Eisenhower, Henry Ford II and Douglas Fairbanks Jr., who have been awarded honorary British knighthoods.

In a brief, private ceremony at Buckingham Palace, Queen Elizabeth dubbed Weinberger a Knight Grand Cross in the Civil Division of the Most Excellent Order of the British Empire in recognition of his "services to Britain," particularly during the Falklands war.

The honor entitles Weinberger, 70, to add the letters "G.B.E." at the end of his name, but not to be called Sir, a privilege reserved for British subjects. Nevertheless, he declared himself "very honored and very pleased and still very astonished."

In impromptu comments outside the palace after the ceremony, Weinberger declined to confirm that he had bowed on receipt of the honor, saying only that he "did what proper protocol called for." Whether to bow when introduced to British royalty has in the past been a subject of controversy for Americans, some of whom feel that such obligations were rejected once and for all in 1776.

After prompting from photographers, a pleased-looking Weinberger opened the blue presentation box to display the insignia of the Order of the British Empire, a gold cross, and the silver star of the Knight Grand Cross. He is entitled to wear them, according to protocol, on "appropriate occasions."

He was accompanied by his wife Jane, who leaned heavily on a cane and was assisted in and out of the building by her husband and a palace aide. It was her illness that led to his resignation and retirement last November after seven years as President Reagan's defense chief.

Widely known here as an ardent Anglophile, Weinberger was a frequent visitor to Britain during his years with the administration. He was given the award on the special recommendation of British Foreign Secretary Geoffrey Howe, a knight since 1970.

In a statement, the foreign office described Weinberger as a staunch friend to Britain "who will be remembered most of all for his unfailing support and assistance during the Falklands War." Although the level of U.S. assistance to the British effort against Argentina in 1982 was kept largely secret at the time for fear of offending Latin American sensibilities, Weinberger is known to have sent London communications intercepts and other intelligence on the Argentines, as well as resupply of U.S. weaponry.

Prime Minister Margaret Thatcher gave a dinner in honor of the Weinbergers last night, attended by Prince Charles. Today's ceremony took place in the Queen's Audience Room, where she receives Thatcher in a weekly audience, and occasionally other important visitors. Principal among the paintings on the walls are two Gainsboroughs of King George III and Queen Charlotte, under whose reign the American colonies were lost.

There are somewhere between 3,000 and 4,000 knights extant. According to the government Central Office of Information, "the honor of knighthood takes its form from the usages of medieval chivalry, from which also comes the method still normally used in Britain of conferring a knighthood by the touch of a sword (the 'accolade') by, or on behalf of the sovereign."

The G.B.E. is the most senior of five degrees in the Order of the British Empire, which was founded in 1917, "chiefly to recognize service by civilians in the first world war," and is "now the most widely conferred on civilians or service personnel for public services or other distinctions."

Honors bestowed by the monarch usually are awarded on the recommendation of the government of the day, which uses them to recognize achievement as well as a form of patronage for partisan service.