Russians Alexander Petrov and Ruslan Boshirov (not thought to be their real names) are alleged to have carried out a nerve agent attack in Salisbury in March.
Ex-Russian spy Sergei Skripal, 66, and his daughter Yulia, 33, were poisoned with Novichok on 4 March.
Det Sgt Nick Bailey also fell ill after responding to the incident in Salisbury.
Police linked the attack to a separate Novichok poisoning on 30 June, which led to the death of Dawn Sturgess.
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