AMSTERDAM (REUTERS) - Blood samples taken from Kremlin critic Alexei Navalny confirmed the presence of a nerve agent from the banned Novichok family, the global chemical weapons watchdog said on Tuesday (Oct 6).

The Organisation for the Prohibition of Chemical Weapons (OPCW) said in a statement that the biomarkers in his blood and urine had "similar structural characteristics as the toxic chemicals belonging" to the Novichok group.

The findings confirm results released by Germany, where Navalny was treated after falling ill on a flight in Siberia on Aug 20. Berlin asked the OPCW to take samples from Navalny and test them after German doctors concluded he had been poisoned with Novichok.

The precise substance in Navalny's samples that was detected by the OPCW's designated laboratories was not on the list of banned chemical weapons, but is a new and undeclared variant in the Novichok family, the statement said.

"These results constitute a matter of grave concern," said OPCW director-general Fernando Arias, calling on members to uphold the international treaty banning chemical weapons use.

Western governments have called for sanctions against Moscow over the case. Russia denies any involvement and has said it doubts Navalny was poisoned.

"No doubt Novichok nerve agent used to poison Alexey #Navalny," Britain's delegation at the OPCW said on Twitter.

A joint statement from 44 OPCW member countries including the Germany, Britain and the United States called on Moscow to investigate.

"We urge the Russian Federation, on whose territory the attack took place, to investigate and to disclose, in a swift and transparent manner, the circumstances of this chemical weapons attack," the statement, tweeted by the British delegation, said.

Novichok, a Soviet-era nerve toxin, was also used to poison a former Russian spy in England in 2018.

The OPCW's member states agreed last year to ban chemicals in the Novichok family, a ban that went into effect four months ago.