AS WELL as making a seminal contribution to the study of Anglo-Norman England, the Oxford historian John Prestwich worked at Bletchley and the Pentagon during the Second World War.

Born in Leigh, Lancashire, in 1914, John Oswald Prestwich went to school at Sedbergh. Thereafter his life was spent very largely in Oxford: as an undergraduate at Hertford, a prize fellow of Magdalen, a tutorial fellow of Queen’s from 1937 to 1981, and then in fruitful retirement in Old Headington.

When war broke out, the brilliant academic career that had been predicted since his undergraduate essay won the university’s Lothian Prize went into abeyance. In Hut 3 at Bletchley Prestwich was part of the team with the crucial responsibility for interpreting decoded intercepts and on the basis of these drafting clear, concise and accurate signals to be sent to the commanders in the field, particularly in North Africa. Whether he was writing lectures or articles, his penetrating analysis of texts and his mastery of lucid and precise prose remained a Prestwich trademark to the end.

In 1944, as Major Prestwich, he was sent to Washington, and for the rest of the war he worked at the Pentagon, dealing with intelligence relating to Japanese ordnance and troop movements. After the role of Ultra became common knowledge, he talked with infectious enthusiasm about his time at Bletchley, but generations of his pupils from the 1950s to the 1970s had not been given the slightest inkling of it.

Although deeply sceptical about many things, Prestwich was always a strong believer in the virtues of the Oxford tutorial system. When he returned to Queen’s and to teaching history and political thought, he shouldered an immense teaching load, but he was unstinting of his time both in tutorials and when it came to writing references and letters of congratulation or consolation.

He was a dauntingly good undergraduate teacher, taking his pupils directly to the primary sources and expecting them to ask questions of them as tough as those he himself posed — both to the sources and to his students.

Books and maps would litter the floor of his study as he and pupils reconstructed medieval political and military campaigns. Through his research students in posts in universities in Britain and Ireland he came to have a very wide influence on the teaching of medieval history. When former pupils published work of their own, they invariably had the genuine pleasure of receiving letters in which their efforts were, as of old, meticulously discussed, and suggestions for improvement were made.

In 1938 he married Menna (née Roberts), another young historian. She became a fellow of St Hilda’s, and for many years the two of them were a formidable force in the Oxford history faculty, sharing values and views, both devoted to their pupils, their colleges, to Oxford gossip, and above all to each other.

Until she died in 1990, it sometimes seemed that Prestwich cared more about the progress of her research into 17th- century French history than his own; he always liked to use comparative material gleaned from his wife’s researches — and their son’s — to reinforce his own.

Yet over a period of 40 years from 1954 he published a number of important articles which established the centrality of the royal military household in the rule of the Anglo-Norman realm and the importance of money in the politics of 11th and 12th-century England. He particularly enjoyed the fact that he, the former tutor, was able to reverse the usual order of things by contributing an essay, “Military Intelligence under the Norman and Angevin Kings”, to a Festschrift in honour of Sir James Holt.

His demonstration that the Latin word rumor, almost universally translated as “rumour”, could also mean “report” enabled him to point out that the “revolution inaugurated by Bletchley’s crypt- analysts in 1940 was a revolution in degree rather than in kind”. There was personal experience too in his view that historians had underestimated the significance of intelligence-gathering in the past because “sensible intelligence services seek to keep quiet about their activities”.

He is survived by his son Michael, also a distinguished medieval historian.

John Prestwich, historian, was born on June 26, 1914. He died on January 25, 2003, aged 88.