The international chemical weapons watchdog has backed Britain’s findings on the type of chemical used in the Salisbury nerve agent attack.

The Organisation for the Prohibition of Chemical Weapons (OPCW) announced on Thursday its findings “confirm the findings of the United Kingdom relating to the identity of the toxic chemical that was used in Salisbury”.

The OPCW said the chemical used was of “high purity”, but said the identity of the chemical, and its name, would only be addressed in the classified section of the report. 

The Foreign Secretary, Boris Johnson, said the findings proved Russia was behind the poisoning of Sergei Skripal, a former double agent, and his daughter Yulia last month. 

The Government has said that its analysis by military experts at Porton Down laboratory showed they were affected by Novichok – a military grade nerve agent developed by Russia.

In response, Johnson said: “This is based on testing in four independent, highly reputable laboratories around the world. All returned the same conclusive results.

“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.”

The poisoning of the Skripals in Salisbury last month, where they were found slumped and unresponsive on a bench in the city centre, has triggered a diplomatic standoff between the west and Russia.

The Kremlin has denied any responsibility for the attack, and has questioned every detail of the UK-led investigation.

After the OPCW’s findings were made public, the Russian embassy tweeted to question the chemical weapons attack in Syria, illustrated with a picture of a black cat.

It also shared what it claimed were letters of support from “many friends”.

Yulia Skripal, 33, was discharged from hospital on Monday and reportedly ushered to a secret location. Her 66-year-old father was still in recovery but doctors said he would be released “in due course”.

Detective Sergeant Nick Bailey, a police officer who responded after they had collapsed and was contaminated with the substance, was released from hospital last month. 

On Wednesday, Yulia Skripal issued a statement saying she did not want consular assistance from the Russian Embassy. 

Yulia also asked her cousin Viktoria Skripal, who has questioned the British Government’s actions and been rejected for a visa to enter the UK, not to contact or visit her.

She said: “I want to stress that no one speaks for me, or for my father, but ourselves.

“I thank my cousin Viktoria for her concern for us, but ask that she does not visit me or try to contact me for the time being.”

The 33-year-old said she has found herself in a “totally different life” as she continues to recover from the attack.

The Russian Embassy said Yulia Skripal’s statement, issued by the Metropolitan Police, “only strengthens suspicions that we are dealing with a forcible isolation of the Russian citizen”.

She said in her statement: “I have specially trained officers available to me, who are helping to take care of me and to explain the investigative processes that are being undertaken.

“I have access to friends and family, and I have been made aware of my specific contacts at the Russian embassy who have kindly offered me their assistance in any way they can.

“At the moment I do not wish to avail myself of their services, but, if I change my mind I know how to contact them.”

Russia’s Foreign Ministry said Britain should prove that poisoned former double agent Sergei Skripal and his daughter are not being held hostage.

Spokeswoman Maria Zakharova said that the Skripals were isolated by British authorities and no one has seen them for over a month.

Zakharova added there were doubts that a recent statement by Yulia Skripal was authentic.

On Thursday, the head of government intelligence agency GCHQ used his first public speech to warn that the Salisbury nerve agent poisonings shows how “reckless” Russia can be.

Ex-MI5 agent Jeremy Fleming said: “For decades, we have collected intelligence on Russian state capabilities, on their intent and posture.

“And for over 20 years, we’ve monitored and countered the growing cyber threat they pose to the UK and our allies.

“This has never gone away. But nevertheless, recent events are particularly stark and shocking.”

The OPCW’s findings follow a setback for Britain’s claims Russia was behind the attack, after scientists at Porton Down scientists tested the agent used on them and confirmed it was Novichok, but said they could not say where it originated.

While Porton Down’s findings did not conflict with the Government’s claims, it proved an embarrassment for Boris Johnson who had said: “People from Porton Down, they were absolutely categorical.

“I asked the guy myself I said ‘are you sure?’ and he said there’s no doubt.”