The Home Secretary has accused Russia of using Britain as a "dumping ground" for nerve agents, as he called on Moscow to explain the Wiltshire poisonings.

Sajid Javid said it was "completely unacceptable for our people to be either deliberate or accidental targets" or for "our towns to be dumping grounds for poison", after a couple were struck down by the same Novichok used in the Salisbury attack.

Charles Rowley, 45, and his 44-year-old girlfriend, Dawn Sturgess, remain critically ill after being exposed to the substance. They were reportedly left frothing at the mouth and hallucinating.

One theory under investigation is that they handled a container - such as a phial or syringe - used to transport the nerve agent for the initial attack on the Skripals and discarded in a public place.

Sergei Skripal, the 66-year-old former Russian spy, and his daughter Yulia, 33, were the target of a suspected Kremlin-backed assassination attempt in Salisbury in March.

Amid fears the public could still be at risk, Ben Wallace, the security minister, earlier confirmed the "working assumption" is the couple taken ill in Amesbury - around eight miles from Salisbury - were not targeted victims, but encountered the substance accidentally. Novichok can be inhaled as a fine powder, absorbed through the skin or ingested.