Baron Caccia of Abernant, 84, who as Sir Harold Anthony Caccia served as British ambassador to the United States from 1956 to 1961, and later became permanent undersecretary of state at the Foreign Office, died Oct. 31 at Builth, Wales. The cause of death was not reported.

Lord Caccia, who entered the Diplomatic Corps in 1929, was permanent undersecretary of state at the Foreign Office from 1962 to 1965, and head of the Diplomatic Corps in 1964 and 1965.

He gained a reputation over the years as a tireless and effective diplomat who handled some of the most challenging assignments of his era with tact and aplomb. He also was regarded as a sportsman and a diplomat with the attributes of staying power and rare common sense.

In July 1956, Britain recalled its ambassador to the United States to take up a new post. It was not until Nov. 9, 1956, that Lord Caccia presented his credentials as the new British ambassador.

Those weeks were among the most important in modern Anglo-American relations and momentous in postwar diplomacy. On the one hand, the Hungarians had risen in violent revolt against the Communist regime, eventually to be crushed by Soviet troops and tanks. While this was happening, Britain, France and Israel launched an in vasion of Egypt after that country's nationalization of the Suez Canal.

This country, like Britain, did little or nothing to aid the Hungarians. In the midst of a presidential campaign, Dwight D. Eisenhower announced that America would not support its two World War II allies in their war with Egypt.

The allies lacked both supplies and money. The invasion ended with the allies withdrawing and the Egyptians winning a political victory.

The Suez adventure led to the near collapse of the pound sterling and eventually to the resignation of an ailing and bitter Prime Minister Anthony Eden.

Lord Caccia, who presented his credentials three days after the U.S. presidential election, began his tenure in Washington during what was probably the low point of postwar Anglo-American relations. He later said his arrival here was "like a paratrooper, firing from the hip."

A champion of the "special relationship" between Britain and America, he summoned reserves of goodwill and memories of a grand alliance, calling on Americans to remember what made the two countries allies and to soldier on in the mutual struggle against communism. He ably represented his country at State, on Capitol Hill and even on the social circuit.

Upon retiring from the Foreign Office in 1965, he served until 1977 as provost of Eton College, responsible for the day-to-day operation of that ancient institution. He also was a director of several companies, including ITT Ltd. of Britain.

He received his first honor from the crown in June 1945, when he was named a companion of St. Michael and St. George.

In January 1950, he became a knight commander of the order. He was created a life peer in 1965.

Lord Caccia was born in India, where his father served in the government, and was a graduate of Eton.

In 1927, he graduated from Oxford University's Trinity College with second-class honors in philosophy, politics and economics. After serving as a third secretary at the Foreign Office, he was posted to the British legation in Peking.

From 1936 to 1938, he was an assistant secretary to Foreign Secretary Anthony Eden. He served with Eden during trying times, when the future prime minister was trying to come to grips with problems surrounding fascist Italy and Nazi Germany.

In the early days of World War II, Lord Caccia was a member of the legation in Athens, and was charge' d'affaires when Italy and Germany invaded Greece. Later during the war, Lord Caccia gained a lasting rapport with another future prime minister, Harold Macmillan, then British resident minister at Allied Headquarters in North Africa. After the fall of Italy, Lord Caccia became British diplomatic adviser to the Allied Control Commission in that country.

In November 1944, he went to Greece as political adviser to the British commander-in-chief, serving in Athens during the near civil war in December and having his jeep shot out from under him.

After assignments at the Foreign Office, where he became deputy undersecretary of state for administration, he was named British high commissioner in Austria in 1950, and ambassador to Vienna in 1951. He held both posts until 1954. During those years, he helped negotiate the peace treaty that restored Austria's independence and resulted in the withdrawal of Allied occupying armies, including the Soviets, from Austrian soil.

In February 1954, he was named deputy undersecretary of state for economic affairs. During the next two years, he accompanied Foreign Secretary Eden and Prime Minister Winston Churchill to several international conferences.

In February 1955, Lord Caccia took part in the first meeting of the Southeast Asia Treaty Organization in Bangkok.

Survivors include his wife, Anne Catherine, whom he married in 1932, and two daughters.