A previously overlooked entry in a 15th-century manuscript has been uncovered, providing vital new evidence that one of medieval England's most influential figures was, contrary to belief, a saint. Thurstan, who served as Archbishop of York from 1114 to 1140, was previously thought to have been passed over for a sainthood; however, the document – a service book from Pontefract Priory – lists St Thurstan in a calendar of saints' feast days observed at the monastery.

Discovered by English Heritage Senior Properties Historian Dr Michael Carter in the archives at King's College Cambridge, the manuscript is written in Latin and its entry for 6 February reads in translation: 'Death of Saint Thurstan, archbishop of York, year of grace, 1140.' Notably, it has been written in red ink – a sign of its importance and significance to the monks at that time.

St Thurstan was a figure of international influence, involved in the affairs of not just the church but also the state – at the highest level.

A great supporter of religious reform, Thurstan played a key role in the foundation of many of Northern England's greatest monasteries, many of which are now in the care of charity English Heritage, including Furness Abbey, Gisborough Priory, Kirkham Priory, Rievaulx Abbey and Byland Abbey.

Thurstan was also politically astute, protecting the North of England from invasion by mustering the English army that defeated the Scots in the Battle of the Standard in 1138 and bringing Canterbury's claim of precedence over York to an end, after many years of dispute.

Dr Michael Carter, Senior Properties Historian at English Heritage, said: 'Thurstan is well known among medieval historians and scholars as a figure of immense political and social significance during the early half of the 12th century, but all have denied that he ever achieved sainthood. The entry in this manuscript is unambiguous proof that Thurstan was indeed a saint and that his name should be seen alongside other religious contemporaries in Northern England, including St William and St Aelred of Rievaulx, St Waldef of Kirkham and Melrose and St Godric of Finchale.

'Born in Normandy in 1070, Thurstan visited the Abbey of Cluny as a young man and vowed to one day become a Cluniac monk. On 25 January 1140, aged nearly 70 and in failing health, he fulfilled this vow by resigning from his position as Archbishop of York and retiring to the Cluniac priory at Pontefract. He died less than two weeks later on 6 February, and, as befitted his status and importance, was buried before the high altar at Pontefract Priory.'

Professor Janet Burton, professor of medieval history at the University of Wales Trinity Saint David, is a specialist in medieval monastic and religious orders and wrote Thurstan's entry in the Oxford Dictionary of National Biography.

She added, 'Dr Carter's discovery has added an extra dimension to our understanding of Thurstan's legacy and his place in the religious culture of the medieval north. A man of European dimensions, Thurstan spent the first five years of his period of office on the Continent where he enjoyed contact with popes and cardinals, and the leading lights in new emergent monastic movements. He was imbued with all the latest reforming ideas that were sweeping the Church. He transformed his vast diocese, introducing administrative change, fostering pastoral care, and above all encouraging new monastic foundations.'

Whilst Dr Carter's research is the first proof of Thurstan's sainthood, other sources give several indications of his sanctity. A few days after his death, the archdeacon of Nottingham experienced a vision of Thurstan in a dream, confirming that he was in heaven among the saints.

Meanwhile several sources say that, after some time, the monks at Pontefract opened Thurstan's tomb and found that neither his body nor the vestments in which he had been buried had decayed and that a sweet smell emanated from the grave. An incorrupt body and the accompanying 'odour of sanctity' were sure signs of sainthood in the medieval period.

Read more about St Thurstan's life here.