The Russian whistleblower who exposed the country's secret chemical weapons programme has revealed the horrific torturous effect of the Novichok nerve agents on their victims.

Vil Mirzayanov described the use of the lethal toxins as a 'brazen' attack by Vladimir Putin, who 'thinks he can use everything to kill enemies'.

Mr Mirzayanov says a large dose of Novichok 'paralyses' victims before 'it causes convulsions, you can't breathe and after that you die'.

The exiled scientist shocked the world in 1992 when he revealed that promises by the Soviet Union to reduce its chemical weapon stockpiles were hollow.

He worked in the top-secret Moscow laboratory where a new generation of even more potent poisons was being perfected.

These gruesome chemical weapons, named 'Novichok' after the Russian for 'newcomer', were designed to be even more lethal than VX or sarin.

Vil Mirzayanov (pictured) described the use of the lethal toxins as a 'brazen' attack by Vladimir Putin, who 'thinks he can use everything to kill enemies'

Soldiers wearing protective suits work at a site in Winterslow, near Salisbury, where a former Russian spy was attacked with a nerve agent

The investigation into the poisoning has led to a series of locations around Salisbury being sealed off and decontaminated

At the time, one former top Soviet military adviser described them as 'political weapons', adding: 'They have a powerful moral and psychological effect.'

Shockingly, they can be created from common, unrestricted and undetectable industrial and agricultural chemicals available worldwide.

As a result, weapons inspectors fear other rogue nations, including Syria and North Korea, could have their own lethal stockpiles of the powerful nerve agents.

Speaking from his home in New Jersey last night, Mr Mirzayanov, 83, described the top-secret laboratory as a 'criminal enterprise'.

Vil Mirzayanov described the use of the lethal toxins as a 'brazen' attack by Vladimir Putin (pictured), who 'thinks he can use everything to kill enemies'

'It's a brazen attack,' he said. 'Putin thinks he can use everything to kill enemies. They don't tolerate any opponents.

'They should be punished. It's an open demonstration of this Russian terrorism.

'The Russian government is telling people who are thinking about revealing more secrets that they can expect the same fate.'

Asked how the nerve agent works, he added: 'It's for paralysing people, it causes you convulsions and you can't breathe and after that you die. If you get enough of a dose of it.

'It's real torture, it's impossible to imagine. Even in low doses the pain can go on for weeks. You cannot imagine the horror, it's so bad.'

The Novichok family of nerve agents were secretly developed over two decades at a research facility 50 miles outside the Russian capital.

Many times more potent than other better known chemical weapons, Novichok agents can render gas masks and protective equipment useless.

Sometimes described as 'gases' they are in fact liquid, intended to be delivered as a fine spray.

A series of poisons, known as Novichok 5, 7, 8 and 9 to identify them, were produced amid conditions of complete secrecy.

They all kill the same way. By inhibiting enzymes that control nerve receptors in the brain.

One expert said victims simply 'forget to breathe'. A tiny drop, almost undetectable, placed on the skin or inhaled can cause death within minutes.

Describing his work, Mr Mirzayanov said: 'They were normal laboratories, they were not underground or anything. They were testing and developing.

'There were around 1,000 people working on this, it was a big deal. You have to test it on animals and after that you have to study the chemical properties... so many laboratories were involved.'

In 1987, one physicist at the laboratory was saved despite being exposed to the chemical when a ventilator stopped working. Witnesses described how he staggered out of the room, describing seeing bright hallucinations before collapsing and being rushed to hospital by the KGB. He was left with permanent injuries after being critically ill for ten days and unable to walk for six months.

Anti-terror police also took over a Sainsbury's car park in Salisbury as part of the rapidly expanding chemical attack probe

Last night experts described nerve agents such as Novichok as second only to the 'atom bomb' as the most deadly weapons ever made. They said that because the ingredients were so common, the poison was ideal for use in an assassination, as well as a weapon of mass destruction.

Pharmacology expert Professor Gary Stephens said: 'This is a more dangerous and sophisticated agent than sarin or VX and is harder to identify. It causes a slowing of the heart and restriction of the airways, leading to death by asphyxiation.

'One of the main reasons these agents are developed is because their component parts are not on the banned list. It means the chemicals that are mixed to create it are much easier to deliver with no risk to the health of the courier.'

Professor Robert Stockman, of the University of Nottingham, said traces of nerve agents did not linger. He added: 'These agents react with water to degrade, including moisture in the air, and so in the UK they would have a very limited lifetime. This is presumably why the street in Salisbury was being hosed down as a precaution – it would effectively destroy the agent.'

Russia went into denial early today, with a former FSB chief claiming scientists at Porton Down had poisoned ex-spy Sergei Skripal and his daughter Yulia.

A top Moscow chemical weapons expert echoed the charge while also alleging nerve agent novichok did not exist.

Moscow was scrambling today to respond to Theresa May's demand for an explanation for the Salisbury attack by midnight.

In an extraordinary blast, Vladimir Putin's predecessor as FSB secret service chief Nikolay Kovalyov claimed Porton Down government laboratory - which identified Russian-made novichok as the agent that poisoned the Skripals - was likely to have been behind the attack.

'There is a laboratory (near Salisbury), which is famous all over the world,' he dismissively told Britain.

'Check if anything leaked from there.'

Sergei Skripal (left) and his daughter Yulia (right) have been in a critical condition since they were found unconscious on a bench outside a shopping centre in Salisbury on March 4

Meanwhile, chemical weapons scientist Dr Anton Utkin - a former UN inspector in Iraq - denied the existence of novichok, and echoed the claim that Britain maybe responsible for the poisoning.

He led the destruction of chemical weapons in Russia.

'I was dealing with elimination of Russian chemical weapons and with all the responsibility I can say that we have never had a weapon under this name,' he said.

Actually, it is very strange that British experts have found out the formula of this 'super secret' poison nobody knows about.

'It is not clear either how they managed to determine that it was produced in Russia.

'If you know the technology, it is not possible to find out who made a chemical weapon – if the method is the same, the formulas of the substances will be identical.

'So this news only provokes more questions.'

Utkin said the West had earlier made a 'toxic noise' about Russia's alleged use of chemical weapons in Syria.

'And now right in the heart of Great Britain this traitor is killed with a neuroparalytic substance.

'All this happens at a very convenient time – before the Russian presidential election and World Cup.

'If I were British, I would not blame Russians but check my own security systems first.

'Something strange is happening there – it is not the first time when criminals are walking around the country with highly toxic and radioactive substances and the secret services have no idea about it.'

Branding the attack a 'reckless and despicable act' last night, Mrs May said the substance used was a 'military grade' nerve agent Russia has produced

Dr Utkin has worked for the State Scientific Research Institute of Organic Chemistry and Technology which is suspected of being behind the development of novichik starting in 1973.

Kovalyov, now an MP, said the accusations by May were a 'provocation' against Russia.

There was no sense for Russia seek to kill a spy convicted 12 years ago when the publicity would damage the campaign for the presidential election, he said.

'Just be logical. There is no logic here,' he said.

'England is turning into a dangerous country,' he said.

'If I were to speak for the Ministry for Foreign affairs, I would have issued a statement that would not recommend our spies and traitors to live there because it is dangerous to these citizens,' he said.

'The chain is already rather long.'

He complained that Britain along with the US and Ukraine see Russian as 'guilty a priori - the Russophobic mood is extreme, all politicians are pursuing their careers with the help of these tendencies.'

The incident was timed to harm Putin in Sunday's election - 'this is the purpose of this provocation', he said.

Skripal 'was not dangerous for Russia.

'He could have told about methods of recruiting but those were from 20 years ago.

'Just look, by the way, how England treats people who gambled everything they had in order to work (for the British).

Skirpal 'lived in some tiny rustic town, with no good job, no proper accommodation.

'Everything around him is so poor, and they threw him a bone from time to time. Nothing to envy.

'And finally, the risk of being killed, poisoned, hanged.

'To imagine that a group of Russian spies is walking around and poisoning traitors at in England, is ridiculous.

'In all other countries they live happily.'

Kovalyev headed the FSB until he was succeeded by Putin in 1998.