New evidence has been discovered of a lost English saint.

An entry in a 15th century manuscript has provided “unambiguous proof” that Thurstan, who was Archbishop of York from 1114 to 1140, achieved sainthood when he was previously thought to have been passed over, according to English Heritage.

English Heritage senior properties historian Dr Michael Carter discovered the previously overlooked entry in a service book from Pontefract Priory, which is in the archives at King’s College Cambridge.

The book lists St Thurstan in a calendar of saints’ feast days observed at the monastery.

The manuscript is written in Latin and its entry for February 6 reads in translation: “Death of Saint Thurstan, Archbishop of York, year of grace, 1140.”

This has been written in red ink which English Heritage said is a sign of its importance and significance to the monks at that time.

St Thurstan was an internationally important medieval figure who played a key role in the foundation of many of northern England’s greatest monasteries, including Furness Abbey, Gisborough Priory, Kirkham Priory, Rievaulx Abbey and Byland Abbey.

He also mustered the English army that defeated the Scots in the Battle of the Standard in 1138, English Heritage said.

Dr Carter said: “Thurstan is well known amongst 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.”

Professor Janet Burton, professor of medieval history at the University of Wales Trinity Saint David, wrote Thurstan’s entry in the Oxford Dictionary of National Biography.

She said: “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.”

English Heritage said Thurstan was born in Normandy in 1070 and died on February 6 1140, aged 70.

Two weeks before his death, he fulfilled a vow he made as a young man to become a Cluniac monk by resigning from his position as Archbishop of York and retiring to the Cluniac priory at Pontefract.

He was buried before the high altar at Pontefract Priory.

A few days after his death, the archdeacon of Nottingham said he experienced a vision of Thurstan in a dream, confirming that he was in heaven among the saints, English Heritage said.

Several sources say that the monks at Pontefract later opened Thurstan’s tomb and found that neither his body nor his vestments had decayed and that a sweet smell emanated from the grave.

An incorrupt body and the accompanying “odour of sanctity” were considered sure signs of sainthood in the medieval period, the charity said.