A federal judge has convicted a 73-year-old reputed Chicago mobster who first gained notoriety three decades ago for helping to steal the 45-carat Marlborough Diamond from a London jewellery store.

Arthur "The Brain" Rachel grimaced, turned to one of his lawyers and shook his head slightly after Judge Harry Leinenweber found him guilty in the bench trial on three of four counts, including racketeering for his involvement in several planned robberies.

Co-defendants Joseph "The Monk" Scalise, 74, and Robert Pullia, 70, pleaded guilty last week.

Scalise was an accomplice with Rachel in what at the time was seen as a daring, daytime theft of the Marlborough Diamond, which has never been recovered.

The Chicago trial, which was not connected to the jewellery theft, attracted attention partly because of the defendants' advanced ages when they are accused of plotting robberies several years ago. Rachel did not speak to reporters as he left the courthouse on Thursday.

Among the evidence taken to court was a battered stepladder the would-be robbers intended to use in a 2010 burglary at the home of late Chicago mobster Angelo "The Hook" LaPietra. Authorities arrested the pensioners, dressed in black, outside the house.

They had also planned to rob an armoured car, prosecutors said.

Scalise and Rachel were convicted in Britain of using a hand grenade as a threat in 1980 to rob London's upmarket Graff Jewellers of 3.6 million dollars (£2.3 million) worth of goods, including the diamond. They began serving 15-year prison terms in 1984 and were released in 1993.

The FBI has said Scalise's CV included serving as a technical adviser on the movie Public Enemies about Depression Era gangster John Dillinger, which was filmed in Chicago in 2008.

Asked by reporters as he left court last week if the Marlborough Diamond could ever be located, Scalise said: "If Lloyd's wanted to pay enough money, maybe they could."