The Prison Service has launched a probe into claims that one of the Millennium Dome diamond theft gang went on a fishing trip with some of his jailers shortly before his trial started.
Kevin Meredith, 35, of Auckland Road, Brighton, was jailed for five years for his part as the getaway driver in the failed £200 million heist.
He is reported to have accompanied warders on two boating trips while on bail before the case was heard.
The News Of The World said he took six jailers he had met while previously held on remand at Belmarsh high security prison on his father's 30ft launch on November 4.
He went on another trip with five warders on his father's second boat on January 12 this year, the paper reported.
A Prison Service spokesman said: "This is a very serious matter. There will be a thorough investigation."
Meredith was last week cleared of conspiracy to rob the Dome but convicted of conspiracy to steal.
He had been hired at the last moment to take four Dome raiders in a speedboat across the Thames if the heist on November 7, 2000 succeeded.
Instead the raiders ran straight into a police ambush after the biggest Flying Squad operation in history.
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