The gruesome reality of chemical experiments carried out by the Ministry of Defence at a controversial 'military science park' over the course of 50 years has been revealed.

Teenager soldiers and servicemen unwittingly volunteered to be human 'guinea pigs' for a series of experiments at Porton Down, in Wiltshire, in the hope of a bit of extra cash.

But the unsuspecting volunteers were exposed to Sarin gas, anthrax and even the Black Death, a new book has revealed.

One victim was left convulsing with ‘terrible stuff coming out of his mouth like frogspawn’; another teenage serviceman believed he had a four-hour conversation with a school-friend who had died years before, after being injected with a brain-incapacitating drug.

Incredibly, thousands of members of the British public were also unknowingly exposed by government scientists who released spores of a 'plague-like' bacteria on the London Underground in 1963.

Eerie: Volunteers line up to be experimented on at 'military science park' Porton Down, in Wiltshire. Scientists exposed volunteers to potentially dangerous gases to measure how much was absorbed by the masks

Controversial: Protesters outside Porton Down, in Wiltshire, where the Ministry of Defence carried out chemical and biological experiments on 21,000 servicemen between 1939 and 1989

Although considered harmless at the time Bacillus globigii - or BG to use its military moniker - can in fact cause food poisoning, eye infections, and even potentially deadly septicaemia.

But none of the London commuters were ever told of the experiment.

Scientists at Porton Down assured their thousands of military volunteers that they were 'totally safe' before exposing them to a series of dangerous experiments.

Despite being turned into ‘guinea pigs’ by their own government, 21,000 servicemen between 1939 and 1989 were only offered token payments, a day off, or even just a free bus pass.

Decades later, in 2008, the government finally apologised for the atrocities that had been carried out on human ‘guinea pigs’ and paid compensation to 670 of the victims.

Historian Ulf Schmidt, a leading academic of modern history at the University of Kent, has revealed exactly what the shocking experiments entailed and the horrific effects they had on their ‘participants’.

The historian, who acted as an expert witness in the Porton Down investigation, has revealed his findings in a new book 'Secret Science: A Century of Poison Warfare and Human Experiments.

Here are some of the most disturbing case studies he has revealed.

Many thousands of Londoners were put at considerable risk by the Ministry of Defence experimentation.

On July 26, 1963, a harmful virus was unleashed on the London Underground.

Spores of the virus Bacillus globigii were released at Colliers Wood, in a tiny box disguised as a make-up compact.

Unknowing test subjects: On July 26, 1963, scientists released spores of the harmful virus Bacillus globigii on the London Underground (right) to test how anthrax spores would travel. The spores were unleashed at Colliers Wood, in a box disguised as a make-up compact (left)

Scientists were trying to discover whether ‘long distance travel of aerosols’ on London’s transport network ‘was due to transportation within trains’ or through the air ventilation systems.

The virus, although considered harmless at the time, has since been proved to cause food poisoning, eye infections and even potentially-deadly septicaemia.

But none of the commuters dusted with the spores were ever warned or contacted afterwards.

Government officials decided that in order to maintain national security, the trial should be kept under wraps.

'Guinea pig': RAF engineer Ronald Maddison, 20, unwittingly volunteered to be exposed to Sarin gas, a deadly nerve agent now classified by the UN as a weapon of mass destruction. He died in May 1953

Ronald Maddison, an RAF engineer from Co Durham, was just 20 when he signed up for one of the hundreds of experiments carried out at the ‘military science park’ of Porton Down, in May 1953.

Assured that he was in no danger, he was guided into a gas chamber with five other test subjects, dressed in oversized overalls, woollen hats and respirators for protection.

Scientists applied twenty drops of liquid to two layers of cloth used in uniforms, serge and flannel, which had been taped to the inside of his forearm.

Just hours later the hapless volunteer was dead, the victim of the most severe case of nerve gas poisoning ever recorded in the western world.

He had been exposed to Sarin, a deadly nerve agent that is now classified by the UN as a weapon of mass destruction.

Ambulance driver Alfred Thornhill, who was 19 years old at the time of Maddison’s death, spoke as an eyewitness at the inquest into his death.

‘I had never seen anyone die before and what that lad went through was absolutely horrific,’ he told the inquest.

‘It was like he was being electrocuted, his whole body was convulsing.

‘The skin was vibrating and there was all this terrible stuff coming out of his mouth…it looked like frogspawn.’

He added: ‘I saw his leg rise up from the bed and I saw his skin begin turning blue. It started from the ankle and started spreading up his leg.

‘It was like watching somebody pouring a blue liquid into a glass, it just began filling up.

‘It was like watching something from outer space and then one of the doctors produced the biggest needle I had ever seen.

‘The sister saw me gawping and told me to get out.’

In payment for the trial that killed him, Maddison was offered 15 shillings and three-day leave pass. He had planned to use the money to buy an engagement ring for his girlfriend.

The British government finally consented to an inquest into his death, half a century after the event.

In 2004 the inquest decided that he had been ‘unlawfully killed’ by his own government, and two years later his family received £100,000 in compensation.

Unwitting volunteers: The chemistry laboratory at 'military science park' Porton Down, in Wiltshire. An total of 21,000 servicemen volunteered to take part in experiments between 1939 and 1989, designed to test chemical and biological weapons capabilities

Airman Richard Skinner, a 19-year-old from Aberdeenshire, arrived at Porton in mid-1972 in the hope of earning some extra cash.

He volunteered himself for what was then described as a ‘mild dose of anaesthetic’.

He was actually injected with the new drug T3436, which had been designed as a means to incapacitate the human brain.

At a hearing, 30 years later, he described how his only hazy recollection of the experiment was a long conversation with a dead school-friend.

In video footage of the experiment, the young Skinner can be seen talking to a fire extinguisher for more than four hours.

He is, to this day, convinced that the mind-altering experiment fundamentally changed his personality.

Thousands of servicemen were locked into gas chambers pumped full of potentially lethal toxins, in a series of experiments designed to test protective clothing carried out over the decades.

In one of these life-threatening experiments, in March 1943, six servicemen were exposed to nitrogen vapour for an hour a day for up to five consecutive days.

But all six volunteers had to be removed from the test, after suffering agonising chemical burns to their armpits, scrotums and scalps.

A 20-year-old corporal, Harry Hogg, was one of those exposed to the poisonous gas.

He later spoke out about his experience in the chamber.

Gruesome: Scientists at Porton Down ushered thousands of servicemen into gas chambers pumped full of potentially lethal toxins over the decades, to test protective clothing

‘It seemed like an eternity. They opened the door and we all piled out on hands and knees, groaning and moaning and crying… one man was just like an animal.

‘He was trying to eat grass. He was out of his mind. What we went through was horrendous.’

In 1944, a Porton Down report insisted that most of those involved in the experiment enjoyed their experience.

In February 1995, Labour politician Rachel Squire demanded that the Ministry of Defence investigate Harry Hogg’s case specifically, as Porton Down bosses tried to deny that he was ever there.

Anthrax was also a key feature of Porton Down experiments, as scientists investigated its capabilities for biological warfare, along with venereal diseases and the bubonic plague.

Porton Down scientists launched Operation Cauldron in 1952, to test the potential of the bubonic plague as a weapon.

The trial took place in the Outer Hebrides, aboard the HMS Ben Lomond.

Trial-run: Scientists investigated the bubonic plague as a biological weapon, aboard the HMS Ben Lomond (pictured) in the Outer Hebrides in 1952, as part of Operation Cauldron

But in an unexpected twist the trawler Carella, with 18 people aboard, strayed into the test area.

Rather than warn or offer treatment to those aboard, researchers were ordered to track the trawler as it continued on its course to Iceland, to monitor what happened.

Incredibly, the crew were even allowed to dock for a few days at Blackpool, with no regard for public safety.

If any of the crew members became sick, medical officers had been instructed to diagnose pneumonia.