The life and crimes of 60-year-old McLean were always coloured by lurid claims of how he had been allowed to build up his empire in exchange for betraying rival drug gangs to the police and intelligence services.

His last will and testament to his family about how MI5 agents had helped him to flee from jail has triggered a feud between rival wings of Britain’s security apparatus.

Customs officers are demanding to know why MI5 apparently assisted the man they blame for the death of one of their colleagues.

McLean’s version of events is that he was given a fake identity by his handlers, then sent to Ireland to infiltrate Britain’s biggest heroin gang. Nine weeks later he was found dead in a scruffy benefits hostel in South London after, it is said, double-crossing MI5 and just hours before he was due to escape for a new life beyond the reach of British justice.

A new book, Cut-Throat, written by McLean’s nephew, Wayne Thallon, uses the crime boss’s own diaries and documents to detail his life as an informer.

The fear is that any public investigation into McLean’s links with police, Customs and the intelligence services could deeply embarrass Whitehall.

Customs agents have told The Times that they are appalled at allegations that MI5 organised the disappearance of McLean only five years into his 21-year sentence for a drug-smuggling operation in which Alistair Soutar, a much-decorated undercover agent, died.

Mr Soutar, 47, was among the team who ambushed McLean’s boat off the coast of Scotland in July 1996, and was crushed to death as he attempted to board the vessel while the crime boss tried to destroy the three tonnes of cannabis on board.

His fellow agents are also angry at the apparent reluctance of their own officers to demand a full, public investigation. The Prison Service has yet to fully explain why McLean, a category-A convict, was moved so quickly to Leyhill open prison in south Gloucestershire, which has the worst record in Britain for absconders.

He lied to the authorities about wanting a transfer to be closer to his wife, who had just bought a home in Edinburgh. On a Saturday in November 2003 McLean was allowed out of prison for a day release.

A van-load of MI5 agents are alleged to have been waiting for him. They are said to have given him a fresh set of clothes. a new identity and fake documents and slipped him on to a ferry to Ireland.

McLean travelled on to Wexford on the southeast coast to make contact with fellow smugglers, who would help him to get in touch with a notorious London-based crime family.

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