A lab run by the Russian security services is among just a handful of facilities which could make a 'very rare' nerve agent which poisoned a Russian spy, it was claimed today.

The huge police team now investigating the attack on former double agent Sergei Skripal are hoping the chemical make-up of the toxin used will show where it was manufactured.

Chemical weapons experts say the sarin or sarin-like nerve agent used in the attack was probably made in a state-run laboratory somewhere in the world.

One of the few labs capable of producing a nerve agent like that used in the attack on a Russian double agent in the UK is in Russia's foreign intelligence headquarters, it was claimed today

Police extended a cordon around Sergei Skripal's house in Salisbury as they test the agent

Police forensic officers in gas masks at a car garage where Mr Skripal's car is believed to have been taken after it was overparked following his attack

One of the labs in the frame is the Yasenevo facility run by Russia's foreign intelligence service, the SVR, according to The Sun.

The SVR is the successor to the KGB and is the Russian equivalent of MI6. It's most famous former agent is Vladmir Putin himself, who was an agent in East Germany in the 1980s.

Hamish de Bretton-Gordon, ex-commander of the Army's chemical, biological, radiological and nuclear regiment, told the Mail: 'This was quite clearly a very professional operation.

'These things are made in big government laboratories. It reduces in toxicity over time so if it was made in Russia two weeks ago then by the time it was administered its effectiveness would be reduced significantly.'

He added: 'Nerve agents cannot be cooked up in a shed. An agent needs to be made in a well-funded laboratory by a highly-trained team of scientists.'

The Home Secretary has meanwhile described the chemical as 'very rare'.

Detectives are understood to be moving away from the theory that the nerve agent was sprayed directly at Skripal, a source told MailOnline.

They are instead believed to be focusing more on the possibility that poison was added to his food or drink at some point before he collapsed.

Investigators hope that by working out exactly which nerve agent was used in the attack on Sergei Skripal and his daughter Yulia (pictured in Salisbury two years ago), they can determine where it came from

Police in Hazmat suits gathered evidence at the bench where the Russians were found

It is hoped the composition of the substance will help investigators work out where the agent was manufactured.

Experts say the clear liquids can be made at only 'a few laboratories in the world', mostly government-controlled.

The first modern nerve agents, including sarin, which was released by a Japanese doomsday cult onto the Tokyo subway in 1995, were first devised by the Germans during World War II.

A new generation of the chemical weapons, including VX, were later invented by the British during the Cold War. VX is up to 150 times more deadly than sarin.

The fact that investigators appear to have ruled out those two suggests the nerve agent used last weekend could be a so-called 'fourth generation' agent.

They were developed by the Soviet Union in the 1980s and 1990s and are said to be even more toxic than VX, which has been classed as a weapon of mass destruction by the UN.

These Novichoks, meaning 'newcomer' in Russian, are said to be binary weapons, which mean they contain two harmless chemicals which only become toxic when mixed together in an aerosol or missile. This makes them easier to make, store and transport safely, but may also render conventional anecdotes ineffective.

Photos emerged today of the immediate aftermath of the nerve agent attack in Salisbury

Police in protective suits and gas masks working near Salisbury earlier this week

Professor Malcolm Sperrin, Fellow of the Institute of Physics and Engineering in Medicine, said: 'Symptoms of exposure to nerve agents may include respiratory arrest, heart failure, twitching or spasms - anything where the nerve control is degraded.'

Scientists do not want to say how nerve agents are created, for fear of copycat attacks, but the ingredients are cheap and easy to obtain, although it takes proper equipment to mix the substances safely.

The chemicals can kill within minutes, by disrupting electrical signals through the nervous system which makes it hard to breathe.

People cough and foam at the mouth as their lungs fill with mucus, they vomit, sweat, become incontinent and their eyes run. It is described by experts as 'turning on all the taps'.

Dr Simon Cotton, from the University of Birmingham, said: 'If you have ever seen a fly sprayed it drops on its back and lies with its legs in leg in the air, twitching, this is the result of nerve agents taking hold.'

After a police officer was hospitalised while helping the spy and his daughter, investigations have taken extra precautions when examining the scene

Scientists at the Porton Down Research establishment are trying to work out where the nerve agent used came from

The Korean leader's half-brother, Kim Jong-nam, died within 20 minutes after his face was smeared with VX at an airport in Malaysia last February. The Ministry of Defence has since admitted carrying out animal experiments using the nerve agent, to try to develop an antidote.

VX is one of five main nerve agents and, with another called GF, is a 'G-series' agent usually inhaled or absorbed through the skin. Just a fraction of a drop, absorbed through the skin, can take effect within seconds and 'fatally disrupt the nervous system', according to the Council on Foreign Relations.

The other three 'V-series' agents are sarin, tabun and soman, which are primarily designed to kill their victims through inhalation and were devised by the Germans but never used during World War II.

All five colourless chemicals are sometimes called nerve gas, but this is incorrect, as they are liquid at room temperature.