When Brian Lawrence, a convicted murderer, wrote the words "more heat, less light" in one of his letters from the Isle of Wight's Parkhurst prison, he must have hoped warders monitoring his mail would put his reference down to his former career as a science teacher.
Instead, the security team at the former high-security prison, which once housed some of Britain's most notorious killers, became suspicious that the 67-year-old inmate, who was convicted of murdering a friend of his former lover and hiring a hitman to kill two others, was using a tried and tested method to conceal invisible messages.
Closer examination of the letters showed that, under the guise of routine correspondence, Lawrence had been using lemon juice as an invisible ink to convey details of a spectacular plot to escape the prison – once considered the British version of Alcatraz – using a helicopter.
Investigators found that for several months Lawrence had been diligently drawing maps and using encrypted instructions, including codes hidden in sudoku puzzles, to convey to his accomplices his plan to bring a helicopter into one of the few parts of the prison grounds not protected by reinforced nets.
The plot was uncovered when staff at the prison heated the documents from underneath, causing a chemical reaction in the dried lemon juice which revealed what had been written.
It transpired that Lawrence, who was sentenced to a minimum term of 21 years in 2005 for a campaign of murder fuelled by "insane jealousy", had planned to use the Isle of Wight Festival last weekend as cover because other helicopters were conveying stars to and from the venue.
A joint investigation by the Prison Service and Hampshire police found that Lawrence had planned to fund his escape by selling a quarry he owned in Newbury, Berkshire, for £500,000. He hoped to reach Spain and assume a new identity.
It is understood that Lawrence has been moved to another prison and disciplinary proceedings are being considered against him. None of his accomplices have been arrested.
Helicopters have become the method of choice for criminals seeking to escape. The gangland boss John Kendall and the murderer Sydney Draper were sprung from Gartree prison in Leicestershire in December 1999 by accomplices using a stolen helicopter. Worldwide, 33 helicopter escapes have been recorded, 14 of them in France.
At his trial, jurors heard that Lawrence, from Hermitage, Berkshire, had hatched a number of elaborate plans, one using poisoned tomatoes, to try to kill two men he considered rivals for the affections of his married former lover, Christine Watson. In May 2003, Lawrence broke into the home of Korean War veteran Deryk Cox, 69, who was a close friend of Mrs Watson, and bludgeoned him to death with a hammer before setting fire to the house. He then hired an undercover police officer to carry out two murders for £3,000.
Barry Greenberry, governor of HMP Isle of Wight, an amalgamation of the island's three prisons, said: "I am delighted at the way our security team and Hampshire Constabulary have foiled a possible escape attempt. I am very proud of the operations team who diligently pieced together intelligence to stop a security breach."