AMSTERDAM - New evidence has bolstered a theory that Vincent van Gogh's psychotic break on Dec 23, 1888, may have been set off by the news that his brother, Theo, had become engaged to be married.

Author Martin Bailey writes about the findings, based on a thorough examination of family letters, in his new book, Studio Of The South: Van Gogh In Provence, which will be published on Thursday (Nov 3) in Britain.

The artist cut off most of his ear during a psychotic episode about 12 hours after he learned of the engagement, which is "not something you would do if you welcomed the news, by any means", Bailey said in a phone interview on Tuesday.

In the past, most scholars have credited the mental breakdown to a fight van Gogh had that same day with painter Paul Gauguin, a friend of his. Bailey believes the engagement news to be a much more significant disturbance than the fight, and said van Gogh's fears of abandonment may have been stirred.

"Vincent feared that he would then 'lose' Theo, his closest companion," Bailey wrote in the book. "He was equally worried that his brother might withdraw the financial support which had enabled him to devote his life to art. All this was threatened by the unexpected appearance of a fiancée."

Bailey, a London-based art correspondent for The Art Newspaper and an independent curator who has mounted van Gogh exhibitions at the Barbican Art Gallery and the National Gallery of Scotland/Compton Verney, previously posited his theory in an article for The Art Newspaper in 2009 and in his 2013 book The Sunflowers Are Mine: The Story Of Van Gogh's Masterpiece. But his new book provides more evidence, he said, based on an analysis of previously unpublished family correspondence.

"I'm even more convinced that he did indeed know about the engagement on the morning of Dec 23," he said.

Other van Gogh experts are circumspect about the evidence. Ms Nienke Bakker, a curator of van Gogh paintings at the Van Gogh Museum in Amsterdam - which owns the largest collection of the artist's works - said that it's impossible to know whether Bailey's theory is correct, because the letter that van Gogh received on Dec 23 doesn't exist any more.

"It might have contained the news of Theo's engagement, but this cannot be proven," she wrote in an email. "It is equally possible that Theo only informed Vincent of his marriage plans when he visited his brother in hospital - thus after the ear incident."

Mr Sjraar van Heugten, a van Gogh expert and the author of several books about the artist, said, "It seems fair to me that this may have played a crucial role, but it came on top of the increasingly difficult situation with Gauguin." He added: "I give more weight to that, but the dramatic events were probably the result of several tensions."

Scholars have been puzzling over the details of van Gogh's time in Arles, in Provence, for the 126 years since the artist's death, because while he was living there from February 1888 to May 1889, he experienced both the peak of his artistic career and the beginning of his mental decline. He had his first significant psychotic breakdown there, according to a recent conference by physicians and art historians in Amsterdam earlier this year to discuss the case. It is also where he was hospitalised for the first time.

Bailey observes that, just a half day before van Gogh cut off his ear, he received a letter from Paris that may have contained the news that Theo was engaged to Johanna Bonger, known as Jo. Although he could not confirm the precise contents of that letter, which has been lost, he was able to find evidence that Bonger received a telegram of congratulations on Dec 23 from her older brother Henry - which confirmed that Henry had received the news of the engagement - shortly after receiving the announcement letter from Bonger.

Because Vincent and Henry were both older brothers of the betrothed parties, and had the same social standing within the family, it would seem customary to inform them both at the same time, Bailey reasons.

Even if van Gogh had heard about the engagement on that day, however, Ms Bakker of the Van Gogh Museum said: "The question would remain as to whether it played any part in his self-injury. It is a matter of speculation that cannot be proved one way or the other."