RUSSIAN agents may have used a public toilet as a lab to prepare the Novichok used at Salisbury, it is believed.

Counter-terror cops are probing the theory that hit squads smuggled the nerve agent’s components into the UK and mixed it here.

Forensic teams have discovered low-level contamination on a bench and in toilets in the city’s Queen Elizabeth Gardens. A source said: “Novichok is a liquid agent made of two components.

“The toilets in Queen Elizabeth Gardens are a key location where it could have been mixed, prepared or dumped.

“It’s chilling and definitely a line of inquiry.”

Sergei, 67, and Yulia Skripal, 33, fell ill on March 3 after touching Novichok smeared on their front door handle. Both survived.

Dawn Sturgess, 44, died on July 8 after she was given a sealed box containing a Novichok-filled perfume bottle. Boyfriend Charlie Rowley, 45, found it in the park on June 29.

Investigators believe two teams of agents may have prepared two separate phials.

Police said: “The investigation is ongoing. We are not prepared to discuss any lines of enquiry.”