The international chemical weapons watchdog on Thursday confirmed Britain’s finding that a former Russian spy and his daughter were poisoned last month with a nerve agent, as Russia continues to deny it was behind the attack.

The report by investigators at the Organization for the Prohibition of Chemical Weapons confirms "the findings of the United Kingdom relating to the identity of the toxic chemical that was used in Salisbury."

The organization said the chemical was of “high purity” but did not identify its source. The agency also did not name Novichok as the nerve agent, unlike British Prime Minister Theresa May who cited the Soviet-engineered chemical as the one used in the attack.

Russia has denied all involvement of the attack, arguing Britain has not provided any evidence for its assertion.

POISONED FORMER RUSSIAN SPY SERGEI SKRIPAL NO LONGER IN CRITICAL CONDITION, HOSPITAL SAYS

Yulia Skripal was released from the hospital earlier this week after being poisoned with a nerve agent last month. (AP)

U.K. Foreign Secretary Boris Johnson said OPCW's report proves Britain’s claim that Russia is responsible for the attack.

"This is based on testing in four independent, highly reputable laboratories around the world. All returned the same conclusive results," Johnson said. "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 findings come after Yulia Skripal on Wednesday rejected Russian Embassy assistance as she recovers at an undisclosed location. Yulia, 33, was released from the hospital earlier this week, but her father Sergei, 66, is still recovering. He was upgraded from critical condition to stable last week and is said to be responding to treatment.

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"I am safe and feeling better as time goes by, but I am not yet strong enough to give a full interview to the media, as I one day hope to do," Yulia Skripal said in a statement released by London's Metropolitan Police. "Until that time, I want to stress that no one speaks for me, or for my father, but ourselves."

DAUGHTER OF POISONED SPY SERGEI SKRIPAL TURNS DOWN RUSSIAN HELP

Sergei and Yulia Skripal were found unconscious on a public bench in the southwestern English city of Salisbury on March 4.

In 2004, Sergei Skripal was arrested in Moscow and later confessed to having been recruited by British intelligence in 1995. He also said at the time that he provided information about GRU (Russian intelligence) agents in Europe, receiving over $100,000 in return.

At the time of Skripal’s trial, the Russian media quoted the country's FSB domestic security agency as saying that the damage from his activities could be compared to harm inflicted by Oleg Penkovsky, a GRU colonel who spied for the United States and Britain. Penkovsky was executed in 1963.

In 2006, Skripal was convicted on charges of spying for Britain and sentenced to 13 years. However, he later was pardoned and released in July 2010 as part of a U.S.-Russian spy swap, which followed the exposure of a ring of Russian sleeper agents in the U.S.

Skripal’s wife and son have both died in recent years.

The Associated Press contributed to this report.