Experts from the Organization for the Prohibition of Chemical Weapons (OPCW) confirmed that the banned nerve agent called “Novichok” was found in samples taken from poisoned Russian opposition leader Alexey Navalny, the U.K.’s envoy to the body via his Twitter micro blog on October 6.

“These results constitute a matter of grave concern,” OPCW Director-General Fernando Arias said in a website statement.

The finding could lead European countries to impose sanctions on Russia for violating the treaty banning the use of such weapons, German officials have underlined.