Vladimir Putin (pictured yesterday) claimed 20 countries could have poisoned Mr Skripal

Britain today claimed Russia's response to the poisoning of its ex-spy suggests it is 'nervous' about what evidence over the nerve agent attack will reveal.

It came after the head of Russia's SVR foreign intelligence agency accused the British and US security services of a 'grotesque provocation' over the poisoning of its former spy, warning of a 'second Cuban Missile Crisis'.

Russia is taking its complaints over the Salisbury nerve agent attack to The Hague the day after the Porton Down military research facility said it could not trace the agent's precise source.

An extraordinary meeting of the Organisation for the Prohibition of Chemical Weapons (OPCW) has been called by Russia to 'address the situation around allegations of non-compliance' with the chemical weapons convention made by the UK against Moscow.

But Britain's delegation to the OPCW has branded Russia's decision to call the executive council meeting 'perverse'. It also said Russia's response suggests it is 'nervous' about what the evidence will show.

It came as a message stating that Porton Down had established that the Novichok nerve agent came from Russia was deleted from the Foreign Office Twitter feed.

The tweet, issued on March 22, said: 'Analysis by world-leading experts at the Defence Science and Technology Laboratory at Porton Down made clear that this was a military-grade Novichok nerve agent produced in Russia.'

The director of Russia's foreign intelligence service Sergei Naryshkin (pictured in Moscow today) described Britain's claims that Moscow is to blame for the poisoning as a 'provocation'

Sergei Skripal with his daughter Yulia before they were poisoned in Salisbury last month

Meanwhile the EU today insisted it retained 'full confidence' in the UK's investigation of the attack and said Russia was to blame.

But SVR chief Sergei Naryshkin said: 'Even when it comes to the grotesque provocation with the Skripals that was crudely concocted by the British and American security services, a number of European countries are in no rush to unquestioningly follow London and Washington but prefer to look into what has happened in detail.'

Speaking at a security conference in Moscow, he added: 'It's important to stop the irresponsible game of raising stakes and to stop the use of force in relations between states, not to bring matters to a second Cuban Missile Crisis.'

Mr Naryshkin was referring to the 1962 crisis between the Soviet Union and the United States that brought the world to the brink of nuclear war.

Russia has denied responsibility and demanded access to the British investigation into the March 4 attack on former spy Mr Skripal and his daughter Yulia.

The Foreign Office accused Russia of attempting to undermine the work of the OPCW and engaging in 'another diversionary tactic'.

Mr Putin has claimed 20 countries could have poisoned the Skripals (pictured together)

It comes a day after the head of Porton Down said his scientists have not verified that the nerve agent used in Salisbury came from Russia.

The news, which handed President Vladimir Putin a propaganda coup, today saw ministers scramble to shore up Theresa May's coalition of support against Russia.

The Government insisted the scientific data had only ever been part of the intelligence picture which led Mrs May to point the finger of blame at Russia.

A Foreign Office spokesman said of the OPCW meeting: 'Russia has called this meeting to undermine the work of the OPCW, which, fully in accordance with the chemical weapons convention, is providing the UK with technical assistance and evaluation through independent analysis of samples from the Salisbury attack.

'Of course, there is no requirement in the chemical weapons convention for the victim of a chemical weapons attack to engage in a joint investigation with the likely perpetrator.

'This Russian initiative is yet again another diversionary tactic, intended to undermine the work of the OPCW in reaching a conclusion.'

The Porton Down military research facility said it could not trace the agent's precise source

Gary Aitkenhead, Porton Down's chief executive, dismissed Russian claims that the nerve agent used in Salisbury might have come from the defence laboratory

Today's meeting of the OPCW executive council at The Hague will be held behind closed doors. It comes after Mr Putin last night claimed 20 countries could have poisoned the Skripals.

The head of the Porton Down military research facility said it had not yet been possible to say where the Novichok used in the attack was made.

What is the Novichok nerve agent used against the Skripals? The Novichok nerve agent used against former Russian spy Sergei Skripal and his daughter Yulia is among the most deadly poisons ever created. They were secretly developed by the Soviet Union during the height of the Cold war in the 1970s and 1980s. Communist scientists developed the poison so it would not be able to be detected by Nato's chemical detection equipment. They come in the form of a ultra-fine powder, Novichok is up to eight times more potent than the deadly VX gas. Victims who are poisoned by the powder suffer muscle spasms, breathing problems and then cardiac arrest. There is a known antidote to the nerve agent - atropine can block the poison. But doctors find it very tricky to administer the antidote because the dose would have to be so high it could prove fatal for the person. Novichok poisons are highly dangerous to handle, requiring the expertise of skilled scientists in a sophisticated lab. Dr Vil Mirzayanov, former Chief of the Foreign Technical Counterintelligence Department at Russia’s premiere, was among the team of scientists who helped develop the agent. Advertisement

Gary Aitkenhead confirmed the poison had been produced by a 'state actor', saying: 'It's a military-grade nerve agent which requires extremely sophisticated methods in order to create – something that's probably only within the capabilities of a state actor.'

But he told Sky News it was not Porton Down's role to work out its origin and said the Government relied on 'a number of different input sources' in coming to its assessment that this was highly likely to be from Russia.

Mr Aitkenhead flatly denied bizarre Russian claims the substance could have come from Porton Down itself.

Downing Street last night said Mr Aitkenhead's comments made no difference to the Government's assessment that the attack emanated from Russia, which was based only partly on the evidence from Porton Down.

Sources said allies, who have expelled more than 100 Russian diplomats in response to the attack, had already been briefed on the intelligence assessments that put the blame on Moscow.

But there was irritation in Whitehall at Mr Aitkenhead's 'clumsy' intervention, as ministers privately braced for a fresh wave of Kremlin propaganda.

Last night Mr Putin seized on the admission to call for a full inquiry, saying 'the speed at which the anti-Russian campaign has been launched causes bewilderment'.

He claimed the nerve agent could have been made in any of 20 countries.

The Russian Embassy in London said: 'We understood from the very start that UK Government statements on the nerve agent having been produced in Russia were a bluff. Now this has been confirmed by the head of the secret lab.'

But Salisbury MP John Glen said he had 'absolutely no doubt' about Russia's involvement, adding: 'I don't mind what propaganda emerges from the Kremlin.'

Government sources stressed the chemical analysis from Porton Down was only 'one part of the intelligence picture' in the decision to point the finger at Russia.

Russia has demanded to see Yulia Skripal, the daughter of double agent Sergei Skripal

Mr Skripal and his daughter (left) stand outside the front door with another relative in the UK

A Government spokesman said: 'As the Prime Minister has set out in a number of statements ... since March 12, this includes our knowledge that within the last decade, Russia has investigated ways of delivering nerve agents probably for assassination and as part of this programme has produced and stockpiled small quantities of Novichok; Russia's record of conducting state-sponsored assassinations; and our assessment that Russia views former intelligence officers as targets.

'It is our assessment that Russia was responsible for this brazen and reckless act and, as the international community agrees, there is no other plausible explanation.'

There was irritation at Mr Aitkenhead's decision to give an interview that could allow the Kremlin to sow further disinformation.

One Whitehall source said Mr Aitkenhead had 'not realised what he has said', adding: 'The scientists' job is to determine what the chemical is. Intelligence agencies and other bodies ascertain where it is from. There is no other explanation than this has come from the Russian state.'

No cordon was placed around the house when the investigation first began last month

Ms Skripal is 'improving rapidly' after being exposed to Novichok on March 4 in Salisbury

The row came ahead of a meeting in The Hague today of the executive council of the Organisation for the Prohibition of Chemical Weapons (OPCW) to discuss the Salisbury attack.

The meeting was called by Russia to 'address the situation around allegations of non-compliance' with the chemical weapons convention made by the UK against Moscow.

The Foreign Office dismissed it as a 'diversionary tactic'.

But German politician Armin Laschet, an ally of Chancellor Angela Merkel, said: 'If one forces nearly all Nato countries into solidarity, shouldn't one have certain evidence?

The Salisbury road where Mr Skripal lives, and where he is believed to have been poisoned

'Regardless of what one thinks about Russia, my study of international law taught me a different way to deal with other states.'

Meanwhile, retired Russian lieutenant-general Evgeny Buzhinsky warned that relations between Russia and the West could 'end up in a very, very bad outcome'.

Asked to elaborate, he told BBC Radio 4's Today: 'A real war, worse than a cold war ... it will be the last war in the history of mankind.'

Commenting on his remarks, No 10 said: 'As the Prime Minister has made clear, the UK would much rather have in Russia a constructive partner ready to play by the rules. But this attack in Salisbury was part of a pattern of increasingly aggressive Russian behaviour.'