A "missing" portrait by Vincent Van Gogh could be lurking in a dusty corner of an attic in London, according to a new book.
Van Gogh only lived in Antwerp for a few months
The descendents of Belgian doctor Amadeus Cavenaile say Van Gogh painted the portrait after the doctor treated him for syphilis in 1885.
His family, some of whom live in the UK, believe the painting was either brought to London or buried in a garden in Antwerp during World War I.
If the painting is found, it could net the owner up to £50m.
Van Gogh's Portrait of Doctor Gachet, another of the artist's doctors, fetched £46m when it was sold at auction in 1990.
The claims are made in a new book, The Van Gogh File, by Scottish author Ken Wilkie.
Mr Wilkie tracked down the descendants of Dr Cavenaile after seeing his name in the back of a sketchbook owned by the artist, along with a medical prescription.
The sketchbook is held by the Van Gogh Museum in Amsterdam.
Mr Wilkie told BBC News Online: "From looking at what we found I presumed Cavenaile was a doctor from Antwerp as the sketchbook was from Van Gogh's Antwerp period.
"I was very curious. I got out the telephone book and found the grandson of Dr Cavenaile, who was himself a doctor and had carried on the family business in Antwerp.
"He confirmed to me that his grandfather had treated Vincent Van Gogh."
Wilkie said Dr Amadeus Cavenaile Jnr, who died in the early 1990s, had known about the painting, but that no-one had ever asked him about it.
"He told me that Van Gogh did not have any money to pay his grandfather for the prescription so he painted a portrait of the doctor instead.
"It was something quite a few penniless artists did at the time."
Dr Cavenaile Jnr told Wilkie he could recall the small oil painting of his grandfather from his childhood.
He believed it had been taken to London by his aunt Jeanne and her husband Adolph Kliebs, a Russian Jew, when they fled during World War I. They settled in Muswell Hill, north London.
But the Kliebs' daughter-in-law, who still lives in the capital, said she was certain the painting had been left behind.
Mr Wilkie added: "She knew about the painting but she was sure it had been buried in the back garden of the house in Antwerp."
Wilkie travelled to the house, which was by now a carpet shop, and said he "astounded" the owners when he told them they could be sitting on £50m.
And although they did some digging, the owners could not be persuaded to completely dig up the garden, which was now covered in concrete.
"You can't force people to do something if they don't want to," Wilkie added.
Wilkie said he believed there was a 50% chance the painting was somewhere in London.
He said: "It sounds like a weird and wonderful story, but it is entirely plausible. People just don't realise what they might have lying around."