Investigators in hazmat suits have combed the building where one of the Novichok victim lives, as police step up their search to find the source of the toxic nerve agent.

Ms Sturgess, 44, and her partner, Charlie Rowley, 45, remain in a critical condition after being taken ill on Saturday in Amesbury, Wiltshire.

There has been much speculation around how the couple came across the contaminated item, with police yet to locate the source of the chemical weapon.

One theory is that the pair found the substance in a container or vial or syringe which was used to transport the nerve agent in the attack on former Russian spy Sergei Skripal and his daughter Yulia, before being discarded.

Other reports say that they may have came across it through picking up a contaminated cigarette end.

Mr Rowley has been described as having foraged for goods to fix and sell, and is known to have collected discarded cigarettes.

A government scientist has told the BBC that the item was unlikely to have been left in the open before they touched it, before adding that the Novichok was so toxic it was able to pass through the skin and did not need to be ingested.

Police said they have not ruled out more people falling ill from coming into contact with the substance which was used on Sergei and Yulia Skripal in Salisbury in March.

It has been identified as the same nerve agent used in the Skripal attack, but it is not yet known if it is from the same batch.

Yesterday, forensic investigators in hazardous material suits and gas masks searched John Baker House, a hostel in Salisbury where mother-of-three Dawn Sturgess was living.

They were seen to collect a number of samples after they took one from the outside of the building shortly after 3pm.

At one point one investigator was seen to carry a large blue bin from the property.

Wiltshire residents have been told to expect investigators in hazardous material suits to descend on the scenes, while other sites the couple were known to have visited before being taken ill are behind cordons.

Mr Rowley's flat in Amesbury, where they were both taken ill, also has a heavy, operational presence.

Incident response vehicles and fire engines joined police at his Amesbury home.

Dan Kaszeta, a former chemical and biological weapons adviser to the White House and the Secret Service, told the New York Times that the object police are looking for may be a crucial piece of evidence,

"Were the guys who did this given more than they needed?"

"The container may even have fingerprints on it, it’s a smoking gun,” he said.

"But killers sometimes leave a smoking gun at the scene of a crime."

It is the second nerve agent emergency in four months prompted a diplomatic row, with Home Secretary Sajid Javid accusing the Russian state of using Britain as a "dumping ground for poison".

The Russian Embassy hit back, accusing the Government of trying to "muddy the waters" and "frighten its own citizens".