Amesbury poisoning: Nerve agent Novichok used to 'poison' British couple
British nationals Charlie Rowley, 45, and Dawn Sturgess, 44, are being treated in hospital after being found unconscious on Saturday.
The town is located eight miles from Salisbury, where ex-Russian spy Sergei Skripal and his daughter Yulia were poisoned with Novichok in March.
Counter-terrorism officials have launched a major investigation into whether the Amesbury and Salisbury incidents were linked.
Around 100 police officers are now working on the case, and Government scientists are carrying out further tests on the nerve agent.