Prince Sadruddin Aga Khan, who died on Monday aged 70, was the uncle of Karim Aga Khan IV, the spiritual leader of the Ismaili Shia Muslims, and was himself a philanthropist, collector of Islamic art and the holder of several senior humanitarian posts at the United Nations.
Sadruddin was both the youngest and longest-serving UN High Commissioner for Refugees, taking over the post in 1965 aged 32 and remaining in office for 12 years. He was praised for the way in which the UN handled major refugee crises in Biafra during the Nigerian civil war in the 1960s, and in Bangladesh, Vietnam and Chile.
Sadruddin was a gentle, urbane man, popular among UN staff and international diplomats. He described himself as "having a foot in the East and a foot in the West". Although western in appearance, for small dinner parties with his wife Catherine he liked to wear a jellaba. He was just as easy in the company of Islamic as western leaders and was a friend and tennis partner of the senior President Bush.
After stepping down from the UNHCR in 1977, Sadruddin held a series of other senior UN roles and, in 1981, was the favourite to succeed Kurt Waldheim as UN Secretary-General. But although he obtained more votes in the formal ballot than Javier Perez de Cuellar, he was shot down by a veto from the Soviet Union, which reportedly found him "too western". Ten years later, in 1991, he failed again and the job went to the Egyptian diplomat Boutros Boutros Ghali.
Prince Sadruddin Aga Khan was born in Paris on January 17 1933, the second son of Sultan Mohammed Shah Aga Khan III, the hereditary Imam of the Ismaili sect of Shi'ism. The family traces its bloodline back to the Prophet.
Sadruddin, whose name in Arabic means "defender of the faith", recalled recently: "My father insisted that I learnt the Koran and encouraged me to understand the basic traditions and beliefs of Islam but without imposing any particular views. He was an overwhelming personality but open-minded and liberal."
When he was a child, Sadruddin's grandmother, the Begum, used to recite to him the great epic poems of Persia's turbulent history that she knew by heart.
After graduating from Harvard University's School of Arts and Sciences, he attended the Centre of Middle Eastern Studies before beginning his career in the 1950s as publisher of Paris Review. In 1958 he joined the UN as Unesco Consultant for Afro-Asian Projects and became involved in setting up a project to preserve the Nubian monuments in north-east Africa.
The next year he was appointed head of mission and adviser to the High Commissioner for Refugees, rising rapidly to become the head of the agency in 1965.
After stepping down from the UNHCR in 1977, Sadruddin held a series of other senior UN roles, including coordinator for the UN humanitarian assistance programmes for Afghanistan (1988-1990) and special UN representative for humanitarian assistance for Iraq and Kuwait after the Gulf War in 1991. This last assignment required the finest diplomatic skills, as it involved gaining the agreement of the Iraqi regime to a relief programme for tens of thousands of Shia Muslims trapped in worsening conditions in the marshlands of southern Iraq. Despite Saddam Hussein's deep suspicions of the UN, Sadruddin was able, after some tough exchanges with Tariq Aziz, to negotiate its presence for the first time in the area. On his return, he urged the swift lifting of sanctions.
In November 1991 Sadruddin's diplomatic skills also yielded the release of the British businessman Ian Richter, who had been jailed for life in 1986 on bribery charges. Sadruddin was able, with the support of King Hussein of Jordan, to persuade the Iraqi regime that the time was ripe for such a gesture. Richter left the country on the prince's private jet.
After he stood down from UN duties, Sadruddin became increasingly involved in environmental campaigns. In 1977 he had created a think tank, Groupe de Bellerive, which produced numerous reports on threats to the environment. As a long-time resident of Switzerland, he was particularly concerned about the degradation of the Alps by insensitive tourist development and deforestation. In 1990 he founded the charity Alp Action.
Sadruddin's grandmother, the Begum, had left his father a library of Persian books, mystical texts and astrological treatises, and it was through these that Sadruddin became interested in Islamic art. At his 17th-century home, Chateau Bellerive on the shores of Lake Geneva, he built up a a priceless collection of paintings, drawings and manuscripts dating from the 14th century. In 1997 many items from his collection were displayed in the British Museum exhibition "Princes, Poets and Paladins".
Among many honours, Sadruddin was appointed to the French Legion of Honour, and in 2002 he was appointed KBE for his services to humanitarian causes and the arts.
Sadruddin was married for five years to a model, Nina Dyer, and their divorce in 1962 made headlines. Yet, on the whole, he managed to escape the gossip that dogged many members of the dynasty, avoiding the racehorses, fast cars and diamonds favoured by his half-brother Aly, who was briefly married to the actress Rita Hayworth.
Sadruddin and his Greek-born second wife, Catherine Sursock, whom he married in 1972, were well-liked figures on the Geneva social scene. She survives him.