Russia has called an extraordinary meeting of the world’s chemical weapons watchdog to protest the UK government’s allegation it was behind the poisoning of a former Russian spy and his daughter in Salisbury.

Britain blamed Russia for the attack on Sergei Skripal and his daughter Yulia with a military-grade nerve agent, plunging diplomatic relations between the East and West to their lowest levels since the Cold War.

Moscow fiercely denies any involvement and has demanded to take part in the British investigation.

The meeting of the Organisation for the Prohibition of Chemical Weapons (OPCW) was called by Russia to “address the situation around allegations of non-compliance” with the chemical weapons convention made by the UK against Moscow.

Skripal attack aftermath – in pictures Show all 15 1 /15 Skripal attack aftermath – in pictures Skripal attack aftermath – in pictures Scene of attack Members of the emergency services in hazard suits fix the tent over the bench where Sergei and Yulia Skripal were found unconscious on a park bench in Salisbury in March 2018. Getty Skripal attack aftermath – in pictures Victim - Sergei Skripal The retired Russian colonel and former double agent for MI6 was in a critical condition in hospital for more than two months after being exposed to novichok in Salisbury. He was given refuge in the UK after being jailed in Moscow for treason. Mr Skripal came to Britain as part of a high-profile “spy swap” in 2010 in which four men were exchanged for ten Russian "sleeper agents" in the US. In this image he is speaking to his lawyer from behind bars in Moscow in 2006. AP Skripal attack aftermath – in pictures Victim - Yulia Skripal Yulia Skripal was struck down by a novichok poison alongside her father Sergei. Facebook Skripal attack aftermath – in pictures Scene of attack A police officer stands guard outside a branch of the Italian chain restaurant Zizzi where the pair dined at before falling ill. It was boarded off whilst investigators worked on the building and later found traces of the chemical weapon within it. AFP/Getty Skripal attack aftermath – in pictures Scene of attack Large areas of central Salisbury were cordoned off by police following the discovery of the Skripals. Traces of nerve agent were also found in The Mill pub. PA Skripal attack aftermath – in pictures Victim - Nick Bailey Detective Sergeant Nick Bailey, rushed to the aid of the Russian ex-spy and his daughter who were targeted with a nerve agent. He was hospitalized after aiding them and didn't leave until three weeks after the attack. Wiltshire Police/Rex Skripal attack aftermath – in pictures Police investigation - Skripal’s home Police believe they were poisoned at home, and detectives found the highest concentration of novichok on the front door of Mr Skripal’s house. Getty Skripal attack aftermath – in pictures Theresa May visits scene of attack Britain's Prime Minister Theresa May spokes with Wiltshire Police's Chief Constable Kier Pritchard near where the Skripal's were found. Britain expelled 23 Russian diplomats over the nerve agent poisoning and suspended high-level contacts, including for the World Cup on March 14. Theresa May told parliament that Russia had failed to respond to her demand for an explanation on how a Soviet-designed chemical, Novichok, was used in Salisbury. AFP/Getty Skripal attack aftermath – in pictures Skripal days before attack Sergei Skripal days before he was exposed to Novichok, that has left him fighting for life. ITV News Skripal attack aftermath – in pictures Police investigation - military involvement British soldiers were deployed soon after the attack to help a counter-terrorism investigation into the nerve agent attack. One of the places they were asked to help out with was Skripal's home and it's surrounding. They were asked to remove a vehicle connected to the agent attack in Salisbury, from a residential street in Gillingham. AFP/Getty Skripal attack aftermath – in pictures Police investigation Personnel in protective coveralls and breathing equipment cover an ambulance with a tarpaulin at the Salisbury District Hospital. AFP/Getty Skripal attack aftermath – in pictures Police investigation The investigation extended to the grave of Sergei Skripal's son Alexander in London Road cemetery. Getty Skripal attack aftermath – in pictures Police investigation The Counter Terrorism Policing Network requested assistance from the military to remove a number of vehicles and objects from Salisbury. EPA Skripal attack aftermath – in pictures Home Secretary visits scene of attack Home Secretary Amber Rudd visited the scene of the nerve agent attack at the Maltings shopping centre on 9 March. Getty Skripal attack aftermath – in pictures Yulia Skripal speaks for the first time Yulia Skripal, speaking for the first time, said she felt lucky to have survived the nerve agent attack in Salisbury which left her fighting for life. Ms Skripal said her life had been “turned upside down” by the assassination attempt. But the Russian national added she hoped to return to her homeland one day, despite the Kremlin being blamed for the attack. Reuters

It came a day after the head of the Porton Down military research facility said his scientists could not trace the nerve agent’s precise source.

Gary Aitkenhead, the chief executive of the Government’s Defence Science and Technology Laboratory (DSTL), said the poison used in Salisbury had been identified as a military-grade novichok nerve agent which could probably be deployed only by a nation-state.

But he told Sky News it was not Porton Down’s role to work out where the agent came from and suggested the government’s conclusion it was highly likely to have come from Russia was based on “a number of other sources”.

Members of the emergency services in green biohazard suits work near the bench where former Russian spy Sergei Skripal and his daughter Yulia were found in critical condition (BEN STANSALL/AFP/Getty Images) (Getty)

Asked about his scientists’ findings, Mr Aitkenhead told Sky: “We in terms of our role were able to identify it as novichok, to identify it was a military-grade nerve agent.

“We have not verified the precise source, but we have provided the scientific information to the government, who have then used a number of other sources to piece together the conclusions that they have come to.

“It’s our job to provide the scientific evidence that identifies what the particular nerve agent is. We identified that it was from this family and that it’s a military-grade nerve agent. It’s not our job to say where that actually was manufactured.”

The location of manufacture “can be established through a number of different input sources which the Government has access to,” he said, adding: “From our perspective, scientific evidence is only one of those sources, and it requires a number of other things to verify that.

“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.”

A police officer stands behind cordon tape in an alleyway which has been blocked off near the home of former Russian intelligence officer Sergei Skripal in Salisbury (Reuters)

Porton Down’s identification of the substance as novichok was a key point in the evidence presented by the UK in Theresa May’s successful bid to recruit international support in the dispute with Moscow, resulting in the expulsion of more than 100 Russian diplomats from over 20 countries.

Citing Mr Aitkenhead, Russia’s president, Vladimir Putin, called for a thorough investigation into the poisoning during a visit to Turkey, where he said “the speed at which the anti-Russian campaign has been launched causes bewilderment.”

The DSTL stressed on Twitter its evidence was only part of the intelligence picture and said: “It is not, and has never been, our responsibility to confirm the source of the agent.”

Police offers at the crime scene of the nerve-agent attack in Sailsbury (Getty Images)

A spokesman for the Foreign Office said: “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.”

A Downing Street spokesman 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, as well as a new and dangerous phase in Russian activity within the continent and beyond.”