We have had measures in place to prevent stress related illness for a number of years. We take it very seriously

Ministry of Defence

The group have all served in recent conflicts such as Bosnia, The Gulf War, the Falkands, or Northern Ireland.

It is the biggest action of its kind faced by the MoD, and could cost the ministry hundreds of millions of pounds if it succeeds.

Solicitor John Mackenzie, who is representing the servicemen, told BBC Radio 4's Today programme that PTSD was a "deeply disabling" condition that had left many of the veterans emotionally scarred.

"The basis of the action is that the MoD and services should have taken better steps to diagnose and treat the condition once they had acquired it.

"It is a condition that has been recognised in one form or another since the First World War, and probably before that."

Sleepless nights

He said the litigation could have implications for the emergency services, whose employees face similar stresses to those of troops in combat.

Soldiers from the Gulf War are among those suing

The claimants include 40 Welsh Guardsmen, who were trapped in the burning shell of the Sir Galahad when it was attacked by Argentinian planes in Bluff Cove during the Falklands conflict.

Fifty of their colleagues died in the attack, and the survivors say many have not worked since, even though they were not all physically harmed.

Some say they have not slept through a whole night since the attack, which killed 50 of their comrades.

The group action is being brought because the servicemen say their stress was not diagnosed, treated or even taken seriously by the ministry.

Recognised disorder

They say evidence collected by the Israeli authorities during the Six Day War in 1967, and US experience of Vietnam veterans, proved decades ago that servicemen can suffer PTSD as a result of their involvement in military action.

But MoD spokesman said the ministry had behaved "in line with contemporary best practice in our treatment of service personnel with suspected PTSD".

He said: "The MoD has given these cases our full consideration and has been in discussions with the claimants' legal representatives to seek a way forward.

"Several lead cases will be identified and should come to court next year. We hope that the court's findings on these lead cases will set the scene for the resolution of the remainder of the outstanding claims.

"We have had measures in place to prevent stress related illness for a number of years. We take it very seriously."

The MoD is expected to argue that in the Falklands conflict and even during the Gulf War, knowledge about PTSD was in its early stages.

PTSD is now accepted by doctors and academics as a genuine disorder, and was recognised by the American Psychiatric Society only in 1980.

Sufferers exhibit a range of physical and mental symptoms, including headaches, inability to concentrate and involuntary tics, and the variety of symptoms shown makes it difficult for doctors definitively to diagnose.