Some say it is a medieval medical manual written in abbreviated Latin and aimed at well-to-do women. Not true, say others: it was written in Hebrew by an Italian physician and clearly shows Jewish women having ritual baths.

Nonsense, others believe: the text was written in Old Turkish, in a poetic style. Or it may have origins in Old Cornish. Or in the Aztec language of Nahuatl, or in Manchu.

When it comes to the Voynich manuscript, a curious and apparently coded 15th-century document now held in the library of Yale University, perhaps the only thing on which academics, cryptographers and enthusiasts can agree is the depth of its mysteries. The beautifully illustrated text appears to have been written in cyphers representing a real language – but what does it mean?

Now a British academic has claimed the manuscript is a type of therapeutic reference book composed by nuns for Maria of Castile, queen of Aragon, in a lost language known as proto-Romance.

In a peer-reviewed paper published in the journal Romance Studies, Gerard Cheshire, a research associate at the University of Bristol, argues the manuscript is “a compendium of information on herbal remedies, therapeutic bathing and astrological readings” focusing on female physical and mental health, reproduction and parenting.

Rather than being written in code, he believes its language and writing system were commonplace at the time it was written, and he claims the document is the sole surviving text written in proto-Romance.

Though some believe the Voynich manuscript to be a hoax, its vellum has been carbon-dated to the early 15th century, and most scholars accept the text is contemporary. It is named after Wilfrid Voynich, a Polish book dealer who bought it in 1912, but much of the history of its ownership is unknown.

Although the meaning of the volume has tantalised experts since it first came to scholarly attention in the early 20th century – it reportedly eluded both Alan Turing and the cold war-era FBI – Cheshire says he unpicked its mysteries in just two weeks “using a combination of lateral thinking and ingenuity”.

Perhaps inevitably, however, Cheshire’s theory has been met with scepticism among medieval experts.

“Sorry folks, ‘proto-Romance language’ is not a thing,” tweeted Dr Lisa Fagin Davis, executive director of the Medieval Academy of America, of Cheshire’s paper. “This is just more aspirational, circular, self-fulfilling nonsense.”

Cheshire insists his work is anything but. “I experienced a series of eureka moments whilst deciphering the code, followed by a sense of disbelief and excitement when I realised the magnitude of the achievement, both in terms of its linguistic importance and the revelations about the origin and content of the manuscript,” he said.

The identification of Maria of Castile “took a lot of working out”, he told the Guardian by email. “But I had already solved the codex, so I applied lateral thinking and reasoning.”

He argues that the manuscript originated on Castello Aragonese, an island castle off Ischia, and that it was compiled by Dominican nuns as a source of reference for the female-dominated court run by Maria of Castile, the wife of King Alfonso V of Aragon. Maria’s great-niece, he notes in his paper, was Catherine of Aragon, the first wife of Henry VIII.

Cheshire claims the document includes images of Queen Maria and her court conducting trade negotiations while bathing. Italic annotations in the text may have been added by her.

Sceptics of the theory say such eureka moments are nothing new. Dr Kate Wiles, a medievalist and linguist and senior editor at History Today, said a new theory on the manuscript’s meaning happened “on a six-monthly basis at least … there have been at least two in the past year”.

She was not convinced by Cheshire’s theory. “It takes liberties with how we understand languages to work,” she said. “He is arguing for a language built of words drawn from lots of places and periods, but together they don’t create something that is convincing as a workable language.”

She added: “One of the reasons the Voynich manuscript is so appealing is because of languages like hieroglyphics and linear B, which were deciphered. But they didn’t come out of nowhere, they were decades in the making and drew on lots of different scholarly expertise. You can’t just have one person saying: ‘I’ve cracked it.’ You have got to have the field, on the whole, agreeing.”

Asked for his response to those who were unconvinced by his interpretation, Cheshire was bullish. “The journal paper has been blind peer-reviewed and verified by other scholars – that is standard confirmation in the scientific arena. There is no need to persuade anyone, as the solution will be used to study the manuscript by linguists and historians in due course.

“Furthermore, there is no ‘interpretation’ involved, as the alphabet, writing system and language have been fully expounded for others to consistently translate of any word, phrase or sentence.”

He is now inviting others to expand on his work and translate the manuscript in full. The mysteries of the Voynich manuscript have not been laid to rest quite yet.