Fake or Fortune, presented by Fiona Bruce and art historian Philip Mould on BBC1, has attributed the painting to the acclaimed portrait artist who died in 2011.
Bruce said: “Freud is a colossus of 20th-century modern art, and challenging his word was something we undertook with some trepidation.”
London-based designer Jon Turner inherited the work from two artist friends, who told him it was an early portrait painted by Freud when he was at art school in 1939. The subject is a man wearing a black cravat.
Experts at auction house Christie’s identified it as a painting by Freud in 1985, but the artist denied it was his work.
However, Bruce and Mould had a breakthrough when they spoke to the artist’s former solicitor, who found a note in her files of a phone conversation with Freud about the painting.
During that phone call in 2006, Freud apparently said he had started the painting, but it had actually been completed by someone else.
For this reason, he would not acknowledge it as his own work. But when experts analysed techniques and materials used in the painting, they declared that it was the work of a single artist.
A panel of three Freud experts said they believed the painting was by the artist himself, likely from 1939.
Mould, who valued the painting at £300,000 or more, said: “It was different from anything we’d taken on until now – we had never had to arm-wrestle with the words of an artist beyond the grave.
“The more I worked on the picture and Fiona was able to add the background with her inquiries, the more I felt confident about it being entirely by Freud.”
Bruce said: “As this investigation progressed we had to investigate Freud the man as much as the painting. He was an extraordinary and controversial character.
“And only by understanding him could we begin to understand why he would deny that a painting of his was in fact by him.”