The couple poisoned by Novichok were exposed to the substance after handling a contaminated item, police say.

Charlie Rowley, 45, and Dawn Sturgess, 44, collapsed at a house in Amesbury, Wiltshire, on Saturday and remain critically ill.

Home secretary Sajid Javid said the nerve agent was the same as that used on ex-Russian spy Sergei Skripal and his daughter in March.

Russia said Theresa May’s government was subjecting them “to hell”.

Mr Javid accused Russia of using Britain as a “dumping ground for poison” after the second incident involving the nerve agent.

But a Russian Embassy spokesman said all allegations of the country’s involvement were “merely speculative”.

The Met said there were around 100 detectives from the Counter Terrorism Network working alongside Wiltshire Police on the “major incident”.

Salisbury District Hospital’s chief executive Cara Charles-Barks said the couple are “extremely unwell” and remain in a critical condition.