Mark Sedwill, U.K. national security adviser, wrote a letter to NATO secretary general, Jens Stoltenberg, to say there was "no plausible alternative explanation" for the attack carried out on March 4 that left former Russian spy Sergei Skripal and his daughter Yulia hospitalized in Salisbury.
"Only Russia has the technical means, operational experience and the motive," he wrote.
Police said the pair likely came into contact with the nerve agent which was identified as Novichok by the Organization for the Prohibition of Chemical Weapons, at Sergei Skripal's home since the highest concentration of the substance was found on the front door.
Yulia was discharged from the hospital this week and is being kept in a safe but unknown location.
Sergei Skripal remains hospitalized but is no longer in critical condition.
Russia has denied having any involvement in the incident and has said that the U.K. carried out the attack to blame Moscow as diplomatic relations between the two falter.
In a statement Thursday, Russian Foreign Ministry spokeswoman Maria Zakharova said the Kremlin "will not take on faith any conclusions regarding the Skripal case until Russian experts are provided with access to the victims themselves, as well as to the materials mentioned in the OPCW's expert report and the entire volume of information available to London."