Sir Patrick Reilly, 90, a retired British career diplomat who had been called "the perfect mandarin" and who had held such senior posts as Joint Intelligence Committee chairman and ambassador to both France and the Soviet Union, died Oct. 6 at a hospital in Oxford, England. The cause of death was not reported.

Sir Patrick held senior Foreign Office posts in tempestuous times.

He was ambassador to the Soviet Union from 1957 to 1960, when Soviet leader Nikita Khrushchev was in his unpredictable prime. He could range from beguiling friendliness to immensely threatening manners in the same interview. Sir Patrick, with perfect aplomb, agreed or disagreed with Khrushchev while treating him as a gentleman.

Sir Patrick was credited with giving British Prime Minister Harold MacMillan, whom he had known for years, sage advice both on Soviet affairs and on methods of dealing with the Soviet leader. The diplomat also was credited with maintaining morale in the isolated Moscow embassy and with more than holding his own in less formal social gatherings with the Soviet leadership.

A highlight of his Moscow years was the visit of MacMillan and his foreign secretary, Selwyn Lloyd, to the Soviet Union for talks with Khrushchev about an East-West summit.

From 1960 to 1965, he served at the Foreign Office. He was a deputy undersecretary of state and did much of the Foreign Office work involved in Britain's 1961 application for membership in the Common Market, which was vetoed by the French.

In 1965, with Britain and France seemingly farther apart than any time since the early 20th century, Sir Patrick was named ambassador to France. Much of the first two decades of his diplomatic career had been devoted to things French, and his embassy showed it. The ambassador seemed to know, or know about, every living French politician. He even charmed President Charles de Gaulle, who when Sir Patrick retired as ambassador said that he had spoken nobly in defense of his country's interests.

Sir Patrick's greatest trouble as ambassador came from his last boss, Foreign Secretary George Brown. The singular Labor politician, lover of drink and extrovert extraordinaire simply did not like Foreign Office diplomats. It turned out he even loathed Sir Patrick's wife, for no apparent reason. After one horrid visit by Brown to the Paris embassy, where, according to the Guardian newspaper, he was given to "loutish and drunken displays," Sir Patrick simply retired from the diplomatic service.

He spent the remainder of his years dividing his time between an Oxford University fellowship and London business interests.

D'Arcy Patrick Reilly, who was born to British parents in India, was a graduate of Winchester. He received a "first" in "Mods and Greats" at Oxford University's New College. In 1933, he took the exam for the Foreign Service and came out first.

His first assignment was in Iran, where he married Rachel Sykes, the daughter of Brig. Gen. Sir Percy Sykes, the legendary authority on Iranian history and culture. In 1939, he was assigned to the Ministry of Economic Warfare, then served as an assistant to the head of the Secret Intelligence Service (MI6). By 1943, he was in North Africa as an aide to the British minister-resident in North Africa, Harold MacMillan.

After serving as head of chancery in the newly opened Paris embassy, he went to Athens as head of chancery and minister during the Greek Civil War. He would return to Paris, from 1953 to 1956, as the embassy's second in command.

Between 1949 and 1953, he did Foreign Office liaison work with the Ministry of Defense. During this time, he wrote a highly critical assessment of H.A.R. "Kim" Philby, who had applied for a senior MI6 post. Sir Patrick's criticisms were credited by some with preventing Philby, who it turned out was a Soviet agent, from receiving the appointment and perhaps going on to become head of the SIS.