More than 20 Plymouth-based Royal Marines suffered skin blistering during a training exercise on an old Soviet Cold War base where chemical weapons were made.
The troops believed they may have been exposed to mustard gas because of their ailments.
But the Ministry of Defence says the illnesses may have been caused by a poisonous wild plant – a giant hogweed which releases a toxic sap which can cause blindness if - goes into the eye as well as painful skin irritation.
The marines were taking part in a training mission on Sazan Island – an Albanian island just off the main coast off one of the world's poorest countries.
It was a Cold War submarine base used by the Soviets which was also home to plants producing chemical and biological weapons.
But now it is uninhabited apart from a small naval base which keeps an eye on smuggling between Albania and Italy.
Royal Marines have taken part in exercises on Sazan for the last three years – but the last operation in September led to several dozen commandos needing medical intervention.
The commandos were part of three companies of troops from Plymouth based 42 Commando who took part in a simulated 19 hour attack on the isle.
Some of the men had been on Sazan for a few weeks before the dawn attack last September.
The 200 men, from J, K and M companies, took part in the assault on ‘smugglers’ hiding out – and who had been seen by members of the Royal Marines Surveillance Reconnaissance Squadron (SRS).
Landing craft brought men ashore on to beaches while others scaled 100 foot cliffs in the night time landing and dawn attack.
Ground explosions were set off – to simulate bombs dropped from RAF fighter jets – although no planes were used in the exercise.
The enemy smugglers were played by British marines and troops brought in from other units.
The commandos entered long and complex tunnels, bunkers and trenches on the island and were also ordered to clear out scores of disused buildings of the enemy.
A military source said yesterday:”No one knows what caused the skin problems. No one has worked it out yet.
“Men were rolling around in grass and in buildings. We thought it may be the effects of old mustard gas.
“We were later told about this giant hogweed plant but we didn’t have a clue.
“We did not feel anything at the time, it was just dirty and dusty. But we spent some time in a maze of dusty tunnels.
“But then little lumps came up on our skin. My lungs were not playing up, it was just on hands and arms.
“The lumps and puss came up in about 24 hours. Everyone was examined by a doctor afterwards when we returned to our ship. We were told to keep it clean and monitor it.
“It did start to go down after a week. It was tender looking and there was puss.”
Another marine said:“We had all been covered up. There was no skin showing, we all wore gloves and were covered.
“Exercises have been taking place on this island for three years. Before any exercise even in Britain the area has a full recce for health and safety reasons and an emergency evacuation plan.
“And on Sazan Island there were parts of it which were cordoned off that we could not go into. There were also buildings we could not go into because they were unsafe. It is quite controlled. There were also a lot of tunnels that were cordoned off as being dangerous areas.”
Deputy Commander of 3 Commando Brigade, Colonel Kevin Oliver, said of the exercise:”The facilities are some of the best training areas the task group has ever used.”
Over the subsequent 96 hours the marines held further assaults on positions 150km away as well as evacuating British embassy staff from a hostile environment.
As part of the Royal Navy’s Response Force Task Group, the Lead Commando Group is sent in from sea by Navy warships into troublespots around the world.
Lt David Kirk, Troop Commander of 5 Troop, K Company, led the initial assault.
He said:”Moving up through the valley was pretty challenging, you’ve got a lot of kit on and the heat, even at night, is considerable.
“Our part of the attack lasted about six hours and, although challenging, all our aims were achieved.
“We knew it was going to be difficult before we started so we mentally prepared ourselves beforehand and just pushed on through.”
A Ministry of Defence spokeswoman said:”We are aware that a number of personnel reported short term illness relating to training on Sazan Island but can confirm that no personnel have reported any ongoing symptoms.
“Every effort has been taken to establish the cause of the illness which is thought most likely to be a plant.
“No further Royal Marine exercise activity on Sazan Island is currently planned. The health and welfare of our personnel are always our top priorities.”