Peter Sawyer, who has died aged 90, was perhaps the most influential scholar of the Vikings and their activities in the last 70 years. His book The Age of the Vikings (1962) radically challenged the current orthodoxy, presenting the Vikings as traders not raiders. Peter did not deny their destructiveness, but he challenged its scale by looking hard at the question of Viking numbers, and at their ships, and by pointing to the destruction carried out by their contemporaries. 

Son of Grace (nee Woodbridge) and Bill Sawyer, Peter was born in Oxford; he had two sisters, Anne and Mary. His father was a tobacconist on Cowley Road. Peter was educated in Oxford, although during the second world war he spent time with relatives in Milford Haven, Pembrokeshire. He was an undergraduate at Jesus College, Oxford (1948-51), before moving to a research studentship at Manchester (1951-53). Thereafter he held posts at Edinburgh, Leeds and Birmingham before returning to Leeds as a lecturer in 1964, becoming reader in 1967 and professor in 1970. Although he was only in Edinburgh for three years, during which time, in 1955, he married Ruth Duncan, he developed a great love of Scotland, buying a house on Mull. He was also a devotee of Yorkshire and its countryside.

His first marriage having ended in divorce, in 1981 he married Bibi Strand, and the following year took early retirement from Leeds, moving to Sweden. Initially he and Bibi lived near Göteborg, where she was teaching. Peter himself took up an unpaid teaching post in the history department, where he influenced a generation of students. Together they moved to Trondheim, where Bibi held a chair in medieval history from 1996 until 2007, when they retired to Uppsala.

Peter also held visiting chairs in Minnesota (1966-67, and again in 1984) and in Berkeley in 1985. During the first of his visits to Minnesota, with the encouragement of the historian Stuart Hoyt, he developed the idea of creating an annual bibliography listing journal articles on medieval studies: this came to fruition in the International Medieval Bibliography, published from 1967 onwards, initially by the University of Leeds.Peter was a great networker. Very early on in his career he made contact with the leading Scandinavian archaeologists of the postwar period. He was equally keen to be in touch with Anglo-Saxon archaeologists, despite the fact that he once stated that archaeology was an expensive way of discovering what we already knew.