The Home Office covered up a catastrophic blunder in which a pair of drug barons tricked a Tory Home Secretary into releasing them early from jail, it is claimed.
Michael Howard got a royal pardon for notorious criminals John Haase and Paul Bennett in return for key insider information on the illegal arms trade.
A former senior Scotland Yard detective claims evidence about the scam was redacted so the gaffe would not be reported.
The fall-out continued for years, former Det Ch Supt Michael Hallowes says, with unrecovered machine guns being used in later high-profile killings.
Gangsters Haase and Bennett fed Customs false details and got middlemen to stash weaponry, the ex-cop writes in his book.
The ruse worked, and in 1996 the then-Home Secretary let the heroin smugglers walk free 11 months into their 18-year sentences.
In 1997, DCS Hallowes launched Scotland Yard investigation Operation Abonar into how deadly weapons, including machine guns, were getting into criminal hands and being used in gang warfare.
He came across another convict trying to use the same tactic as Haase and Bennett to get out of jail, he says.
Registered firearms dealer Anthony Mitchell was arrested for supplying the pair and other gangs with firearms and ammunition.
But when Mitchell went on trial at London’s Old Bailey in 1999, officials demanded all evidence about Mr Howard, Customs and the scam be redacted from prosecution papers, Mr Hallowes claims.
They also instructed six of 10 offences be dropped, he says.
The ex-detective believes this was a bid to prevent the media reporting how a former minister had been tricked.
Mr Hallowes says: “It was a massive shock. All the evidence we put together about the scam was redacted.
“That was because the Treasury Solicitor, on behalf of Customs, warned the Director of Public Prosecutions that it was not in the public interest to expose in a criminal trial the facts that the Home Secretary and Customs had been duped.”
Mitchell got eight years in prison. Haase and Bennett were jailed in 2008 for 42 years for perverting the course of justice.
Yesterday, Mr Howard said of the Mitchell case that he “had no responsibility for, or knowledge of, the matters referred to by Mr Hallowes in the context of that trial”.
On pardoning Haase and Bennett, he said: “I had no alternative other than to respond to the specific request from the trial judge who had heard all the relevant evidence and had reached the conclusion that the pardon was necessary in the interests of justice.”
HM Revenue and Customs and the Home Office did not comment.
Operation Abonar: Inside story of Britain’s biggest gun-running scandal is published by Clink Street Publishing on March 23
