Pierre Chaplais was for 30 years Reader in Diplomatic at Oxford and for a longer period the dominant figure in his field in England. The historical study of the forms of official documents was developed in France, and he was a bridge between the methodical scholarship of the École des chartes in Paris and the pragmatism of English medieval historians.

Pierre Théophile Victorien Marie Chaplais was born in 1920 at Châteaubriant in Loire-Atlantique, grew up in Redon, Brittany, and studied classics and law at the University of Rennes. On the outbreak of war he volunteered for the School of Artillery at Fontainebleau. After the Franco-German armistice in 1940 he was demobilised and returned to his studies at Rennes. In August 1943 he joined the Resistance organisation Défense de la France and four months later he was arrested by the Gestapo and sent to the Buchenwald concentration camp, where he remained until his liberation in April 1945.

After studies at Rennes and Paris, he went to London in September 1946 with an introduction to Professor V. H. Galbraith at the Institute of Historical Research, who persuaded him to register for a London PhD, which he was awarded in 1950 for a thesis on Gascon Appeals to England, 1259 to 1453. This was followed by two volumes of documents relating to the Treaty of Brétigny and to the War of Saint-Sardos, published in the Camden Series of the Royal Historical Society in 1952 and 1954. Most of the material for these works was in the Public Record Office and it was decided that the documents of medieval diplomacy in the PRO should be published with Chaplais as editor. From March 1949 to September 1955, he was employed as an external editor by the record office. The first volume of the Treaty Rolls was published in 1955 and a remarkable selection of Diplomatic Documents, 1101-1272, followed in 1964. Both set a new high standard of textual scholarship and editorial practice.

In 1955 he was appointed lecturer in diplomatic at Oxford and reader two years later. Wadham College elected him to a professorial fellowship in 1964. He was an enthusiastic teacher who trained countless postgraduate historians in the study of medieval documents.

The first fruit of his work on English charters was Facsimiles of English Royal Writs to AD1100 presented to V. H. Galbraith (1957), edited by Chaplais and T. A. M. Bishop. This was to have been followed by a volume of facsimiles of Norman diplomas before 1100, with commentaries; the book was completed but not published because of difficulties with reproduction rights.

There followed a series of virtuoso articles on aspects of Anglo-Norman and Anglo- Saxon charters, which earned him wide respect for the advances in understanding won by his unfailing eye for telling detail in the examination of seals and of original documents. An investigation of how change in the control of a seal could result in a shift of power led him into a detailed study of the position of Piers Gaveston at the court of Edward II, which resulted in an unexpectedly controversial book in 1994.

After the publication of the nearest thing to a diplomatic handbook for later medieval England, English Royal Documents: King John-Henry VI, 1199-1461 (1971), Chaplais’s work largely reverted from diplomatic to medieval diplomacy with a series of volumes on the conduct of diplomacy between English and foreign governments. This was in fulfilment of the plan conceived for the Treaty Rolls many years before. Two volumes of English Medieval Diplomatic Practice (1975, 1982) were followed in 2003 by English Diplomatic Practice in the Middle Ages, the first part of a project that was left incomplete, as the progressive loss of eyesight left him unable to read.

Chaplais became a Fellow of the Royal Historical Society in 1953 (and its literary director, 1958-64). He was elected to the Society of Antiquaries in 1970 and to the British Academy in 1973. In 1979 he was elected a corresponding Fellow of the Medieval Academy of America at the same time as his friend, the leading French diplomatist Robert-Henri Bautier.

He was irrepressibly charming, full of impish fun and humour but he brought great seriousness to his work. He was exacting in his own scholarship and demanded the same high standards in others. He served both his countries with an equal devotion, and medieval studies in both France and England are enormously indebted to him.

He married, in 1948, Doreen Middlemast. She died in 2000, and their two sons survive him.

Pierre Chaplais, Reader in Diplomatic, Oxford, 1957-87, was born on July 8, 1920. He died on November 26, 2006, aged 86