THE HAGUE/LONDON (Reuters) - The lethal poison that struck down a former Russian spy and his daughter last month in England was a highly pure type of Novichok nerve agent, the global chemical weapons watchdog concluded on Thursday, backing Britain’s findings.

Sergei Skripal, a former colonel in Russian military intelligence who betrayed dozens of agents to Britain’s MI6 foreign spy service, and his daughter Yulia were found unconscious on a bench in the cathedral city of Salisbury on March 4.

Britain blamed Russia for the poisoning and Prime Minister Theresa May said that the Skripals had been attacked with a military-grade nerve agent from the Novichok group of poisons, developed by the Soviet Union in the 1970s and 80s.

Moscow denied any involvement and suggested Britain had carried out the attack to stoke anti-Russian hysteria, but Britain asked the Organisation for the Prohibition of Chemical Weapons (OPCW) to check samples from Salisbury.

Testing by four laboratories affiliated with the global chemical weapons watchdog confirmed Britain’s findings and showed that the toxic chemical was “of high purity”.

The OPCW did not explicitly name Novichok in its published summary, say where the poison may have come from or assign blame for the attack. But it did confirm Britain’s analysis about the substance that had been used.

“The results of analysis by OPCW-designated laboratories of environmental and biomedical samples collected by the OPCW team confirm the findings of the United Kingdom relating to the identity of the toxic chemical,” the published summary said.

FILE PHOTO: Inspectors from the Organisation for the Prohibition of Chemical Weapons (OPCW) arrive to begin work at the scene of the nerve agent attack on former Russian spy Sergei Skripal, in Salisbury, Britain, March 21, 2018. REUTERS/Peter Nicholls/File Photo

Testing by OPCW laboratories, the details of which were kept confidential, also found the substance used in Salisbury to be of “a high purity”, which supports the British government’s assertion that a state was involved.

Responding to the summary, Moscow said that it had reason to think the report was part of a British operation to discredit Russia. Foreign Ministry spokeswoman Maria Zakharova said Moscow would not believe any conclusions about the poisoning unless Russian experts were given access to the investigation.

The toxin attack prompted the biggest Western expulsion of Russian diplomats since the Cold War as allies in Europe and the United States sided with May’s view that Moscow was either responsible or had lost control of the nerve agent.

Moscow has hit back by expelling Western diplomats, questioning how Britain knows that Russia was responsible and suggesting it stemmed from a plot by British secret services.

NOVICHOK ATTACK

Russia has said it does not have such nerve agents and President Vladimir Putin said it was nonsense to think that Moscow would have poisoned the Skripals. A police officer was also taken ill after attending the scene.

British Foreign Secretary Boris Johnson hailed the findings. “There can be no doubt what was used and there remains no alternative explanation about who was responsible – only Russia has the means, motive and record,” Johnson said.

A German Foreign Ministry spokesman said the OPCW confirmed Britain’s analysis. “It is now up to Russia to finally play a constructive role and answer the open questions,” he said.

Juergen Hardt, foreign policy spokesman for Chancellor Angela Merkel’s conservatives, said the report provided certainty that the Skripals were poisoned by Novichok.

“This finding massively increases the pressure on Moscow to fully explain the exact course of events and background of the first nerve agent attack on European soil,” he said.

The poisoning of Skripal, who settled in Britain in 2010 after being released by Moscow in a spy swap, shows “how reckless Russia is prepared to be”, the head of Britain’s GCHQ spy agency said on Thursday.

Yulia Skripal, who was released from hospital on Monday, said in a statement she was still suffering effects of the poisoning and her father remained seriously ill. She said she had declined an offer of assistance from the Russian Embassy. [L8N1RO6G7]

Zakharova also challenged Britain to prove the Skripals were not being held hostage, noting that no one except British authorities had seen either of them for over a month.

There are several variants of Novichok, a binary weapon containing two less-toxic chemicals that, when mixed, react to produce a poison several times more lethal than sarin or VX.

Russia’s ambassador to Britain, Alexander Yakovenko, has identified the alleged poison as Novichok A-234, derived from an earlier version known as A-232.

Britain has said the use of such an obscure poison indicates Moscow was either to blame or had lost control over its nerve agents.

“The high purity of the substance will strengthen the UK’s position that the agent was made by a highly proficient team and in a well refined process,” said Alastair Hay, Professor Emeritus of Environmental Toxicology at Britain’s University of Leeds.

The laboratory results, which came in on Wednesday night, were to be circulated to OPCW member states on Thursday. The results will be debated at an emergency OPCW session next Wednesday, to be convened at Britain’s request.