Specialists racing to trace the source of a nerve agent poisoning that has left two people fighting for their lives in hospital are focusing their operations on the victims’ Wiltshire homes.

Dawn Sturgess and partner Charlie Rowley are critically ill in Salisbury District Hospital after coming into contact with novichok, the lab-made poison used to attack Sergei Skripal and daughter Yulia in the city earlier this year.

Fire engines, ambulances and an incident response unit could today be seen on the cordoned-off street in the city where John Baker House, the homeless hostel where Ms Sturgess lived, is located.

Forensic tents have been put up outside the building, which has been evacuated, and neighbours told not to be alarmed by the sight of investigators in hazardous material suits.

Meanwhile, police, ambulance and fire services were also seen carrying specialist equipment such as respirators and protective equipment at Mr Rowley’s house eight miles away in Amesbury.

Other locations known to have been visited by the pair before they fell ill – including a park, a chemist and Baptist church, which have also been cordoned off by authorities – have by contrast been left relatively deserted, although still under police guard.

More than 100 counter-terror officers are involved in the search for clues as authorities attempt to find the source of the poisoning and discover how the pair became exposed to the nerve agent, which was produced in the Soviet Union during the Cold War.

One man watching events unfold outside John Baker House told The Independent he had glimpsed men in military fatigues in the back of a large white transit van, which could be seen just metres from the home.

At the other end of Rollestone Street, police stood guard outside large iron panels in front of three tents erected as part of the ongoing response.

Members of the public appeared bewildered by events as they stood watching on.

One waitress at a nearby fast food restaurant around the corner said she knew Ms Sturgess, who lived at the shelter, describing her as “nice” and saying she often talked to her and her colleagues.

Authorities have insisted that the risk to the public is low, but have also urged people in the area not to pick up discarded items they may find lying about.

It has been suggested that one or both of Ms Sturgess and Mr Rowley may have come into contact with something discarded by the Skripals, such as a cigarette butt, or part of the device used to deliver the poison during the attack.

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

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”.

A spokesperson for the Metropolitan Police, which is coordinating the operation, said the activity was part of the ongoing investigation but declined to provide further details.