Johnson believes the two unprovoked murders he committed are attributable to the stress he suffered while a young soldier in Northern Ireland. He believes that he, and others like him, should be in a separate, specialist prison.
He has been diagnosed as suffering from Post Traumatic Stress Disorder (PTSD) and is appealing against his conviction on the basis of that condition. His appeal argues he should be convicted of manslaughter, not murder. His solicitor, Stephen Sullivan, said: "If the appeal goes through it could open the floodgates to ex-soldiers with PTSD who now languish in the prisons."
PTSD is a syndrome arising out of a horrific experience which later results in flashbacks. They can be triggered at any time. If the original event involved extreme violence, the individual can react violently when the flashbacks strike.
Jimmy Johnson, who was decorated for bravery in Northern Ireland in the 1970s, blames his stress on two incidents. In one, he found the body of a woman who looked like his own wife. The other was a violent struggle in which he nearly beat a man to death with the butt of a gun.
The flashbacks began when Johnson left the Army in 1974. In some, quiet English country lanes suddenly became sniper alleys, and in others he thought ordinary crowds were angry mobs.
One day he was travelling in a van when a ball hit the side of the vehicle. Johnson says he blacked out, flashing back to Northern Ireland. He believed the van to be under attack - and bludgeoned his companion, Keith Culmer, to death. He was convicted of the murder and released nine years later. After 18 months of freedom he was working at a house when he suddenly flashed back to the incident with the gun butt and killed a school laboratory technician with a hammer. He pleaded guilty and was sentenced to life with a recommendation of a minimum 30 years. Johnson is separated from his wife. They have two children.
His experience is not isolated. Ex-soldier Michael King had a flashback that he was under attack at his Suffolk home and fired at passers-by from his window with a shotgun. He pleaded guilty to criminal damage butcited PTSD in mitigation. He got a conditional discharge on condition that he had therapy.
A former Scots Guards sergeant won pounds 100,000 in an out-of-court settlement with the MoD after serving two years for holding members of his company hostage in Londonderry. Heclaimed that PTSD suffered in the Falklands led to his actions.
Historian Alistair Renwick is about to publish a book on ex-soldiers and PTSD. He said: "I believe between 4 and 7 per cent of the prison population is made up of ex-servicemen - between 2,500 and 4,400 men."
An MoD spokesman said: "It is inevitable that some individuals will have left the armed services before they develop the symptoms of PTSD. Responsibility for their health care rests with the NHS, who have given assurances that the needs of ex-service personnel can be met."