Ronald Spring, 70, the chief prosecution witness against his alleged accomplice, rejected suggestions that he had been involved in a string of shady deals - including one to steal another Picasso masterpiece.
Mr Scott, 67, whose victims in the 1960s and 1970s included film stars Sophia Loren and Elizabeth Taylor, is accused of trying to set up a deal to offload the abstract work Tete de Femme, valued at pounds 650,000, which was stolen from a Mayfair gallery. He denies conspiracy to handle stolen goods.
Snaresbrook Crown Court in south London was told that Spring, a former legal executive, had been involved in a plot to obtain a Picasso work, Tete de Mousquetaire, by replacing it with a fake in the London saleroom of Christie's.
Under cross-examination yesterday Spring, of Southgate, north London, said: "I just knew what was being suggested could not happen. It was pure fantasy. I just played along for the entertainment to see what would happen, but I knew all along it was impossible."
The court heard the "fantasy" plot to steal the Christie's Picasso had fallen through just two days before the Tete de Femme was stolen from the Lefevre Gallery by a man brandishing a shotgun on 6 March last year.
But Spring strenuously denied suggestions that he had been involved in the 6 March robbery as part of a series of deals involving works worth millions, including stolen paintings stolen from Longleat, the Marquess of Bath's Wiltshire stately home.
The court had heard previously that the Tete de Femme was recovered only after Spring had unwittingly set up a deal to sell it to a team of undercover detectives posing as crooked art dealers. Mr Scott, who claims he has "retired" from crime to become a celebrity tennis coach, and lives in Islington, north London, told Spring that he wanted up to pounds 75,000 for the Picasso portrait within seven days, the court was told.
It was alleged that he asked for the cash when he handed the work to his alleged accomplice within hours of the Mayfair robbery.
Helen McCormack, for the defence, put it to Spring that he had been the key to the deal after his previous involvement in the art crime world. "The explanation is that you were up to your neck in stealing art for many years, certainly since 1994," she said.
But Spring hit back, saying the Tete de Femme deal, from which he planned to net pounds 25,000, had been the first time he had dabbled in the world of art crime. He told the court: "I am no Al Capone. This is absolute rubbish."
The court heard that Spring, who had already admitted a charge of conspiring to handle stolen goods arising from the Mayfair robbery, told an undercover police officer, named only as Patrick, that the raider was known to him and had a future in art theft. He said: "This young fellow is a rough diamond."
The case continues.