He added: "The plot included from an early stage the kidnapping and holding hostage of Colin Dixon, his wife and young child, who, under threat of lethal violence, gave access to the depot and unimaginable amounts of cash.
"This was, as I said previously, organised banditry for uniquely high stakes.
"It is clear that as a long-term friend and associate of Murray and with knowledge of what was being planned, you played an active part in various aspects of the preparations for the raid."
Allen's plea was also accepted on the basis he was working for his best friend and fellow cage-fighter Lee Murray and "did his bidding" in the planning of the raid.
The pair fled to Morocco in February 2006 and began a cocaine-fuelled party lifestyle in the sun, buying property and expensive cars.
Court documents also revealed they spent thousands of pounds on breast enhancement operations in England for their partners.
Allen went on trial after being arrested and extradited to the UK but the Old Bailey jury was discharged in January after it failed to reach a verdict.
Kent's chief crown prosecutor Roger Coe-Salazar said more of the cash could be recovered
Jurors were told the robbery was masterminded by Lee Murray, who remains in Morocco.
Ian Glen QC, for Allen, said: "For several months this defendant enjoyed the high life in Morocco but it was still very much a question of the boss and the corner man."
Allen had been due to face a retrial at Woolwich Crown Court last week.
Depot manager Colin Dixon and his family were kidnapped at gunpoint by men posing as police officers to allow the gang to gain entry in February 2006.
Fourteen staff members were also tied up at gunpoint as robbers loaded cash into a lorry during the raid.
Following Allen's guilty pleas, Kent Police and lawyers from the Crown Prosecution Service vowed to track down the remaining millions and bring any outstanding offenders to justice.
Kent Police Chief Constable Michael Fuller said: "When I launched this investigation I made it clear that we would go anywhere in the world to find those responsible for this violent crime.
"We have done that and I'm pleased those perpetrators have been brought to justice.
"Allen's guilty plea means an end to this part of the proceedings in this country.
"It does not mean an end to the investigation and by no means an end to recovering the outstanding money or assets obtained through the proceeds of this robbery."
Five people were jailed in January 2008 for their parts in the crime.
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