Theguardian_ A painting that was found by a junk dealer while he was clearing out the cellar of a home in Capri, and was regularly decried by his wife as “horrible”, is an original portrait by Pablo Picasso, Italian experts have claimed.

After he stumbled across the painting in 1962, Luigi Lo Rosso took the rolled-up canvas home with him to Pompeii, where it hung in a cheap frame on the living room wall for the next few decades.

The portrait, which is now believed by its owners to be a distorted image of Dora Maar, a French photographer and painter who was Picasso’s mistress and muse, featured the famous artist’s distinctive signature in the top left-hand corner. But Lo Rosso didn’t know who he was.

It was only much later, when his son Andrea started to ask questions after studying an encyclopedia of art history given to him by his aunt, that suspicions were aroused.

The family eventually sought the advice of a team of experts, including a well-known art detective, Maurizio Seracini. After years of complex investigations, Cinzia Altieri, a graphologist and member of the scientific committee of the Arcadia Foundation, which deals with the valuations, restorations and attributions of art works, confirmed that the signature on the painting, today valued at €6m (£5m), was Picasso’s.

“After all the other examinations of the painting were done, I was given job of studying the signature,” Altieri told the Guardian. “I worked on it for months, comparing it with some of his original works. There is no doubt that the signature is his. There was no evidence suggesting that it was false.”

Picasso was a frequent visitor to the southern Italian island of Capri and the painting, which is strikingly similar to Picasso’s Buste de femme (Dora Maar), is believed to have been produced between 1930 and 1936.

Lo Rosso has since died but his son Andrea, now 60, pursued his quest to discover the artist behind the painting.

“My father was from Capri and would collect junk to sell for next to nothing,” he said. “He found the painting before I was even born and didn’t have a clue who Picasso was. He wasn’t a very cultured person. While reading about Picasso’s works in the encyclopedia I would look up at the painting and compare it to his signature. I kept telling my father it was similar, but he didn’t understand. But as I grew up, I kept wondering.”

Andrea Lo Rosso said there were moments when the family considered getting rid of the painting. “My mother didn’t want to keep it – she kept saying it was horrible.”

He has contacted the Picasso Foundation in Málaga several times, but he said it had shown no interest in examining his claims, believing them to be false. The foundation has the ultimate word on the authenticity of the painting, now stashed in a vault in Milan.

Picasso, who died in 1973, produced more than 14,000 works and the foundation receives hundreds of messages a day from people claiming to be in possession of an original.

The Buste de femme (Dora Maar) was painted in 1938 and stolen from a Saudi sheikh’s yacht in 1999 before being found 20 years later.

Luca Marcante, president of the Arcadia Foundation, believes there may be two versions of the work. “They could both be an original,” he told Il Giorno newspaper. “They are probably two portraits, not exactly the same, of the same subject painted by Picasso at two different times. One thing is for sure: the one found in Capri and now kept in a vault in Milan is authentic.”

Marcante will now present the evidence to the Picasso Foundation.

“I am curious to know what they say,” said Lo Rosso. “We were just a normal family, and the aim has always been to establish the truth. We’re not interested in making money out of it.”