Sir Arthur Watts was one of the most highly regarded - and widely liked - among a talented generation of British international lawyers. He moved from a varied and adventurous career in the Foreign Office into a strikingly successful international practice at the Bar after his retirement. He had the unusual distinction of being the first and only British counsel to have been retained by France to represent it in legal proceedings in the International Court of Justice.

Arthur Desmond Watts was born in London in 1931. His father was a colonel in the Army Education Corps, and early childhood in India was followed by Haileybury and Sandhurst. However, health problems led Watts not to a commission in the Army, but to Downing College, Cambridge, where he began reading economics in 1951, moving to law a year later.

He fell under the spell of international law, as magisterially taught by Arnold McNair and Hersch Lauterpacht, and by his tutor at Downing, Clive Parry. He took firsts in the law tripos and the LLM, won the Whewell Scholarship, and was recruited into the Diplomatic Service in 1956 to join the Foreign Office's small and very select legal staff. Call to the Bar followed at Gray's Inn a year later.

His first big challenge came in 1959, when he set off to Cairo as part of the initial cohort of the British Property Commission set up to administer the re-establishment of commercial and financial relations with Egypt after the Suez disaster and the massive nationalisation of British property and commercial interests that followed it.

After three years in that unique post, it was back to the Foreign Office. The next posting, to the Embassy in Bonn, came in 1967. Much of the Occupation regime was still in place, in West Germany as well as in Berlin, but it was a period of relative stability in East-West relations. Watts and his wife could enjoy their time in a house on the banks of the Rhine which was immortalised by John le Carré in A Small Town in Germany.

After a brief secondment to the Attorney-General, Watts was recalled to the Foreign Office. The renewed negotiations for British entry into the European Communities had begun in earnest, and Watts became a key figure both in the negotiation of the Treaty of Accession itself and in the organisational preparations for membership. He was therefore the logical choice for the first substantive legal adviser to the new UK Representation in Brussels, where his service straddled the completion of the transitional period and the arrival of full membership in 1973, in recognition of which he was appointed CMG in 1977.

A posting back to the Foreign Office brought close contact with the Antarctic Treaty system, which involved meetings in a wide variety of national capitals, and the chance to visit Antarctica. He led for the UK in the long and difficult negotiations over Antarctica, that eventually led to a minerals regime in 1988, bringing skilfully into balance the political impossibility of banning minerals exploitation altogether with the scientific and ecological requirements that made actual exploitation highly unlikely.

In 1987 he was appointed legal adviser to the Foreign and Commonwealth Office. He saw it as his task to help to make and guide policy rather than to criticise from the sidelines. He took silk in 1988, and was appointed KCMG in 1989. He became a bencher of Gray's Inn in 1996 and honorary Fellow of Downing in 1999.

After retirement in 1991, he took a seat in chambers at 20 Essex Street, where he gave the lie to the assumption that skill in advocacy grew only out of independent practice. Despite never having had the opportunity from within the Foreign Office of representing his own country before the International Court, he was almost immediately sought after as counsel by other states, in the end in seven separate cases, and he advised more than two dozen governments.

He pleaded an important part of Nigeria's case before the court in the frontier dispute with Cameroon, and acted for Jordan in the advisory proceedings on the wall in the occupied Palestinian territories. His patient dissection of the arguments that had been raised against the court delivering an advisory opinion in the latter affair is regarded as an international forensic classic.

Representing France, Watts prevented an attempt by Australia and New Zealand to resurrect their case against French nuclear tests in the Pacific.

As his reputation in this field grew, so did the demand for his services as an arbitrator, both in international commercial and investment disputes and in sensitive inter-state disputes. He played a particularly significant role in the lengthy and delicate proceedings of the Ethiopia-Eritrea Boundary Commission. He remained deeply committed to the principle that all states should have ready access to international tribunals, and was always on the lookout for ways in which that could be done within the financial means of even the poorest among them.

Other public activities included shaping the reigning Prince of Liechtenstein's UN initiative which led to the founding of the Liechtenstein Institute on Self-Determination at Princeton. At the request of Carl Bildt, then UN High Representative, he undertook the mammoth task of bringing about a settlement over the assets and liabilities of the former Yugoslavia. There, his firmness of principle and legal adroitness overcame the bitter hostilities between the successor states and brought them almost single-handedly, through years of wrangling, to an eventual agreement.

Jordan made him a Grand Officer of the Order of al-Istiqlal, and he had been due to receive from Slovakia the Dual White Cross a few days after his death.

On top of all this, Watts made a notable contribution to the scholarly legal literature. At an early stage he assisted Lord McNair in producing the 4th Edition of The Legal Effects of War. His Hague Academy lectures on the legal position of heads of state in international law represented the only full-scale treatment of the subject, and were extensively referred to in argument in the Pinochet litigation.

He regularly contributed to the leading legal journals, but his main scholarly achievement was the long collaboration with Sir Robert Jennings on the magisterial 9th Edition of Oppenheim's International Law, the leading international treatise, which appeared in 1992 and might not have seen the light of day at all without Watts's patience and industry.

Watts was seldom without a project. In 1996 he acquired a parcel of land close to Arundel, and an absorbing plan to restore and convert the derelict ancient barn incorporating all that remained of the 12th-century Tortington Priory. The restoration, by Neil Holland, won several architectural awards and led to Watts becoming chairman of Sussex Heritage, as well as providing him with a home that became his favourite place of work.

Cricket was an abiding passion. He had played for his school and college, then in the Minor Counties championship for Shropshire, and later still in Cairo at the Gezira Club. But he counted his greatest achievement the organisation of a multinational match at the South Pole in 1985, with the equipment ferried in at his persuasion by the New Zealand Antarctic survey.

He married, in 1956, Iris Collier, later to become a historical novelist. They had two children. They remained on friendly terms even when in later years their lives moved apart, and he shared at Tortington a devoted partnership with Cecilia Gillette, a former Foreign Office colleague in Brussels.

Sir Arthur Watts, KCMG, QC, barrister and international arbitrator, was born on November 14, 1931. He died suddenly on November 16, 2007, aged 76