News, views and top stories in your inbox. Don't miss our must-read newsletter Sign up Thank you for subscribing We have more newsletters Show me See our privacy notice Invalid Email

It is one of Britain's most secretive sites, remaining shrouded in mystery for more than 100 years.

But this week Porton Down found itself at the centre of one of the biggest diplomatic crises the UK has faced in recent years.

The top secret defence base in Wiltshire was instrumental in helping identify the nerve agent used to poison a former Russian spy in Salisbury.

Sergei Skripal and his daughter Yulia were poisoned with a weapons grade nerve agent in the city last week, leaving them fighting for their lives in hospital.

And, not only have scientists from the specialist laboratory been at the centre of a clean-up operation following the attack, but its helped identify Russia as the source of the poison.

(Image: REUTERS) (Image: Press Association) (Image: Daily Mirror)

What is Porton Down?

The 7,000 acre site, near Salisbury, is one of the UK's most secretive and controversial military research facilities and the oldest chemical warfare research installations in the world.

Scientists from Porton were among the first to create biological weapons as well as one of the world's most lethal chemical weapons, but now its main purpose is to support the military and help combat terrorism.

Porton Down opened in 1916 as the War Department Experimental Station for testing chemical weapons during WW1.

Scientists at the lab researched and developed weapons agents used by the British military during the war such as chlorine, mustard gas and phosgene.

After the war the government decided that work should continue at the site and by 1930 it had grown and developed into the Chemical Defence Experimental Station.

During WW2 research concentrated on chemical weapons like nitrogen mustard and nerve agents such as sarin.

It then led to the development of CS - or tear gas- and VX nerve agent in 1952, one of the most lethal substances ever created which results in a painful death.

(Image: Daily Mirror) (Image: Getty Images Europe) (Image: Daily Mirror)

(Image: Hulton Archive)

The base was later named the Chemical Defence Experimental Establishment.

Chemical weapons were tested on site. Scientists built cannisters full of poison gas that could be released by a timer and they also filled shells with it and released them at targets.

But many of the shells failed to explode meaning the fields are still full of the active chemical agents.

Now Porton concentrates on devising defensive measures against gas attacks after its chemical and biological weapons programme was closed down in the 1950s.

On the government's website it says: "To help develop effective medical countermeasures and to test systems, we produce very small quantities of chemical and biological agents.

"They are stored securely and disposed of safely."

(Image: Getty) (Image: Getty Images Europe) (Image: supplied pixel8000) (Image: AFP)

Human experiments

Since 1916 more than 20,000 people have taken part in studies at the base.

Porton Down's experiments on humans have been widely criticised as it is alleged some human 'guinea pigs' were duped into taking part in tests.

Tests were carried out on servicemen to try and determine the effects of nerve agents on humans - with one recorded death due to a nerve gas experiment.

Leading Aircraftman Ronald Maddison died aged 20 in 1953 after taking part in sarin nerve agent toxicity tests.

(Image: Daily Mirror) (Image: PA Archive/Press Association Images)

During the tests sarin was dripped onto his arm through two layers of cloth. He died shortly after and an inquest in 2004 returned a verdict of unlawful killing.

There are still concerns that the tests have damaged the long-term health of many of the subjects.

From 1945 to 1989 the base exposed more than 3,400 people to nerve gas - testing more human subjects with the chemical than anywhere else in the world.

Tests included going into gas chambers, repeated exposure, mental performance tests and tests on bare skin.

Subjects were monitored to see how their performance would be effected and what psychological impact it had on them.

Other tests included the impact nerve gas had on different parts of the body such as hearing and sight.

(Image: Getty Images Europe)

(Image: Daily Mirror)

The government says that "without their involvement we could not have developed the highly effective protective clothing and medical countermeasures that our armed forces rely on".

Adding: "We still carry out trials with human volunteers to make protective equipment easier to wear and to develop better training procedures.

"These trials comply with all nationally and internationally accepted ethical standards."

Some of the equipment includes better gas masks and the creation of lighter, more bullet-proof body armour.

And speaking of the death of Maddison it said it was a "tragic and regrettable incident".

It said: "He is the only member of the UK Armed Forces to have died as the direct result of participation in experimental tests carried out at Porton Down on behalf of the Ministry of Defence."

Animal testing is also carried out.

Viruses

Initial samples of the Ebola virus were sent to the Porton Down lab in 1976.

The lab now allegedly contains samples of some of the world's most aggressive diseases including Ebola, anthrax and the plague.

(Image: Daily Mirror)

(Image: Daily Mirror)

Aerial release trials

Between 1953 and 1976 a number of aerial release trials were carried out to help the government understand how a biological attack might spread across the UK.

The government said: "Given the international situation at the time these trials were conducted in secret."

And added: "The information obtained from these trials has been and still is vital to the defence of the UK."

Two separate and independent reviews concluded the trials did not have any adverse health effects on the UK population.

(Image: REUTERS) (Image: PA)

Aliens

There has been speculation over the years that alien bodies could be hidden at the sight.

But the government has said: "No aliens, either alive or dead have ever been taken to Porton Down or any other Defence Science and Technology Laboratory site."