David Niven, the former British Army lieutenant whose debonair charm conquered Hollywood and helped make him an Academy Awardwinning star and a perennially popular character actor, died yesterday, a nephew reported. Mr. Niven was 73 years old.
The cause of his death was not immediately disclosed, but he had been suffering from a debilitating neuro-muscular disorder, amyotrophic lateral sclerosis, which is also known as Lou Gehrig's disease.
''My uncle died peacefully and without pain,'' Michael Wrangdah said in Chateau d'Oex, Switzerland, where Mr. Niven died in his Alpine chalet. ''His last gesture a few minutes before he died had been to give the thumbs-up sign.''
Slim, witty and lighthearted, Mr. Niven crowned two urbane decades before the film cameras with his Academy Award as the best actor of 1958, which came for his performance as a fraudulent British major in the drama ''Separate Tables.''
Appeared in 'Wuthering Heights'
He had important roles in many other films, including Samuel Goldwyn's ''Wuthering Heights'' (1939), in which he played the gentle husband of a restless Merle Oberon. And he starred as the adventurous Phileas Fogg in Michael Todd's ''Around the World in 80 Days'' (1956).
Mr. Niven's other major films included ''Dawn Patrol'' (1938), ''Stairway to Heaven'' (1946), ''The Bishop's Wife'' (1947), ''Enchantment'' (1948), ''Court Martial'' (1955) and ''Where the Spies Are'' (1966).
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Mr. Niven's film career began in the 1930's with work as an extra and in bit parts. It then ripened into solid featured roles in more than a score of pictures, including a part as a flirtatious major in ''Dodsworth'' (1936). But it was not until after service as a British Army officer during World War II, that Mr. Niven attained full stardom.
Late in his career, Mr. Niven also gained success as an author. His best-selling volume of memoirs, ''The Moon's a Balloon,'' was praised in 1972 by Christopher Lehmann-Haupt in The New York Times for its ''racy wit and fine sense of the absurd.'' His 1975 nonfiction work, ''Bring on the Empty Horses,'' was praised by William F. Buckley Jr. in The Times as ''a book about Hollywood and incidentally a masterful self-portrait.''
And his best-selling 1981 novel of Hollywood and wartime London, ''Go Slowly, Come Back Quickly,'' was praised in The Chicago Tribune for its ''wonderful anecdotes and escapades.'' Upper-Crust Background
In an industry not known for traditional politesse, he sometimes astounded film critics by writing them thank-you notes after they had praised his work.
Such mannerly flourishes sprang naturally from Mr. Niven's uppercrust background. He was born James David Graham Niven on March 1, 1910, in Kirriemuir, a market and linen-mill center in the Scottish Highlands.
His father, William Edward Graham Niven, was a British officer who died in the Gallipoli campaign of World War I. His French mother, the former Henrietta Etta Degacher, later married Sir Thomas Comyn-Platt, a Conservative Party figure.
The future movie star attended the Stowe School in Buckinghamshire and the Royal Military College at Sandhurst. Commissioned a lieutenant in the British Army, he saw service on Malta before resigning from the Army in 1932 to seek his fortune across the Atlantic. First Film in 1935
His first film appearance of any consequence was in a minor part in the 1935 Samuel Goldwyn drama ''Splendor,'' starring Miriam Hopkins and Joel McCrea. He then had a minor part in ''Rose Marie'' (1936), which starred Jeanette MacDonald and Nelson Eddy.
In 1938, came what some film historians consider to be one of Mr. Niven's best earlier roles, an irrepressible but authentic-seeming airman in ''Dawn Patrol,'' which was a remake of an earlier World War I aviator movie.
The following year, in ''Wuthering Heights,'' Mr. Niven won the praise of Frank S. Nugent, then film critic of The New York Times, for his ''dignified and poignant'' role as Merle Oberon's husband. Later that year, in the little remembered comedy, ''Bachelor Mother,'' he co-starred with Ginger Rogers and Charles Coburn. Back to the Army
Before 1939 was out, Mr. Niven returned to Britain and rejoined the British Army as a lieutenant. He served until 1945, seeing combat and rising to the rank of lieutenant colonel - with time out to appear in the war film ''The Way Ahead.'' For his war service, he was awarded the United States Legion of Merit.
Back in civilian life, Mr. Niven starred as a pilot in the British romantic fantasy film, ''Stairway to Heaven'' (1946), made by Michael Powell and Emeric Pressburger. Bosley Crowther of The Times called his acting ''sensitive and real.''
In the comedy ''The Bishop's Wife'' (1947), Mr. Niven was overshadowed by Cary Grant, but he drew excellent notices, as he did also for his acting in the 1948 love story ''Enchantment.'' During this phase of his career, much of his work was in film comedies, including ''Happy Go Lovely'' (1951), ''The Lady Says No'' (1952), ''Island Rescue'' (1952) and ''The Moon is Blue'' (1953). Appeared on Broadway
In 1951, Mr. Niven made a brief detour to Broadway, starring in the farce ''Nina,'' which had 45 performances. Returning to the movies, he gave what some film historians call one of his finest performances, in ''Court Martial,'' a British drama that drew some enthusiastic reviews when it appeared in 1955 but has since been largely forgotten.
The next year, in the box-office hit ''Around the World in 80 Days,'' Mr. Niven's acting attracted less attention than it otherwise might have, because the cast included many other stars in cameo appearances.
Reviewing ''Separate Tables,'' the 1958 screen adaptation of Terence Rattigan's play, Bosley Crowther said that, as the fraudulent major ''who turns out to be a particularly sad sort of person, David Niven starts weakly and gains strength, so that his final scene of gathering valor is one of the best in the film.''
In later years, Mr. Niven appeared in such comedies as the entertaining ''Please Don't Eat the Daisies'' (1960) and ''The Pink Panther'' (1964), which was the first of the Panther series. More Recent Films
And he won warm praise from critics for his starring role in the 1966 espionage movie ''Where the Spies Are.'' Howard Thompson of The Times said that Mr. Niven gave his ''most appealing performance in several seasons'' and was ''entirely persuasive and convincing as a confused, aging but spunky novice in the cutthroat business of espionage.''
His more recent movies included ''A Man Called Intrepid'' and ''Rough Cut'' in 1979 and ''The Sea Wolves'' in 1980. Mr. Niven also worked in television, serving as host of ''The David Niven Show'' from 1959 to 1964 and starring in ''The Rogues'' in 1964 and 1965, among other activities.
In February, Mr. Niven went to London for treatment of his illness, and his wife, Hjordis, reported that he was very weak and tired. But after nine days in a London hospital, he flew to Switzerland.
His literary agent, George Greenfield of John Farquharson Ltd., said in London Thursday that in the last weeks of his life Mr. Niven had continued working on a romantic-adventure novel titled ''From Dawn Until Dusk.'' Mr. Greenfield said that Mr. Niven had finished three-quarters of the book, which was to be brought out by Doubleday in the United States and Hamish Hamilton in Britain. The agent did not say whether it would be completed, but he reported that in a recent note Mr. Niven had said he was trying to finish it by Thanksgiving ''- whenever that may be!''
In Britain in 1940, Mr. Niven met and married Primula Rollo, then an officer in the Women's Auxiliary Air Force. She died in 1946 after an accidental fall during a game of hide-and-seek at the Hollywood home of Tyrone Power.
In 1948, Mr. Niven married Hjordis Tersmeden, a Swedish model, and they had a house in Cap Ferrat, in Southern France, in addition to the chalet in Chateau-d'Oex, a resort in western Switzerland.
In addition to his wife, Mr. Niven is survived by two sons from his first marriage, David Jr., of London, and James, of New York, and two daughters adopted during his second marriage, Kristina and Fiona.
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