Laying claim to being the first autobiography ever written in English, the "extraordinary" life story of the medieval mystic Margery Kempe, which exists in only one known copy, has been digitised by the British Library for the world to view.

Kempe lived in Norfolk from around 1373 to 1440. After she had given birth to 14 children, she made a vow to live chastely with her husband, and embarked on pilgrimages to Jerusalem, Santiago de Compostela, Italy and Germany. Her devotion was expressed through loud cries and roars, which often irritated bystanders, but she became famous as a mystic, and claimed to have conversations with God.

She dictated her life story to a priest, but her autobiography was only known through excerpts printed by Wynkyn de Worde in 1501, and by Henry Pepwell, who called her a "devoute ancres", in 1521, until a complete manuscript – thought to be a copy made from the original, possibly under Kempe's supervision – was discovered in a cupboard in the 1930s.

"The story goes that when Colonel W Butler Bowdon was looking for a ping-pong bat in a cupboard at his family home near Chesterfield in the early 1930s he came across a pile of old books. Frustrated at the disorder, he threatened to put the whole lot on the bonfire the next day so that bats and balls would be easier to find in future. Luckily a friend advised him to have the books checked by an expert and shortly afterwards Hope Emily Allen identified one as the Book of Margery Kempe," said Sarah J Biggs, from the British Library's medieval team.

Biggs said the memoir, which has just been digitised by the British Library, was "perhaps the first autobiography written in English", and is also "a remarkable record of the religious life of a woman during the tumultuous 14th and 15th centuries".

Kempe refers to herself in her book in the third person as "this creatur", opening her story by telling of how after the birth of her first child she became ill and believed herself to be surrounded by devils. Forcibly restrained, she tore at her skin and bit her hand so hard she retained the marks for the rest of her life, before Jesus appeared to her in a vision and she grew calm.

"Margery Kempe led an extraordinary life," said Biggs, "and this manuscript is of incredible importance for scholars of this period. Prior to the 1930s this text was only known through 16th-century printed versions, and is the first such manuscript containing her autobiography. We are pleased to be able to make it available worldwide."

The first page of the manuscript states "Liber Montis Gracie. This boke is of Mountegrace", with the pages annotated by four scribes, said the British Library, "probably monks associated with the Carthusian priory of Mount Grace in Yorkshire". Their notes highlight key passages, such as "nota de clamore" when Kempe utters her first cry, and "mirabile" to mark a miraculous event.

One of these miracles, said the British Library, occurs when Kempe meets a very learned priest, a "Dewcheman" (German) who could not understand what she said, so they had to speak to each other through an interpreter. Kempe advised the priest to pray for 13 days, and at the end of this "he undirstod what sche seyd in Englysch to hym and sche undirstod what that he seyd. And yet he undirstod not Englisch that other men spokyn".

"A miracle indeed! But for Margery this was not an occasion to celebrate," said the library. Instead, "sche sobbyd boistowsly and cryed ful lowde and horybly".

"One wonders what the learned priest's reaction could have been," said Biggs, adding that Kempe had been described by critics as everything from a religious eccentric to a feminist icon, a literary genius and an early social reformer, but that "however we view her, there is no doubt that the work provides an invaluable insight into 15th-century urban life and into the religious practices of the period."