THE nerve agent used to poison Sergei and Yulia Skripal was administered in liquid form, and does not produce any vapours or gases, government scientists have confirmed.

At a briefing this morning, Department for Environment, Food and Rural Affairs (Defra) confirmed the majority of sites have now been released by the police investigating the incident, and the process of decontaminating them can begin, which will see new cordons erected in the city in the coming days.

Defra scientists believe the contamination levels at "the majority of sites" will be "relatively low", but that the clean-up process will be different for each site depending on how it has been affected.

Sergei Skripal's house, in Christie Miller Road, and the Mill pub in the Maltings still form part of the ongoing investigation and remain under police control, and Mr Skripal's house was confirmed to be the "most contaminated" site.

The clean-up process is designed to "eliminate any residual trace of the agent to remove risk".

Sites classified as "public service" sites, including the Bourne Hill police station and the Salisbury and Amesbury ambulance stations, will see work starting "in the next few weeks", along with the Maltings.

Work is scheduled to start at Zizzi immediately afterwards, followed by DS Nick Bailey's house.

It will take "some months" to return the sites to use, but Defra said they hoped this would be completed by the end of 2018.

The briefing heard that the Novichok had been administered in a liquid form, and a Defra scientist said it had been "a very small amount" as the substance is "very toxic".

Defra confirmed that, in the case of this incident, direct contact with the nerve agent would be required for a person to be poisoned or contaminated.

A chief Defra scientist also said the Novichok would become "diluted" with each secondary or tertiary contact and likened the substance to ink - saying if you put your finger in ink and then touched a number of surfaces, each following surface would have a fainter trace and smaller amount of ink.

And the agent used does not produce "any significant vapour or gas", Defra said, and it can only be moved between sites through a direct transfer through a contaminated person.

And although the agent is "soluble in water and can be washed away", clean-up processes and full tests will still take place at the sites.

Members of the military, police and fire service will be seen in hazmat suits around the city while the decontamination takes place, and cordons at the Maltings will be replaced with semi-permanent barriers, blocking any public view within "the coming days".

Defra warned that some cordons will be increased in size to allow work to be carried out, but said this would take place over a matter of days rather than weeks.

The process involves taking samples from the site and testing them for the nerve agent, removing contaminated items and incinerating them, and then chemical cleaning if it still required.

Sites will then be re-tested before they are deemed safe to open to the public.

Public Health England confirmed that the risk to the public remains low, as any potential sites of contamination have been cordoned off, and that there have been no further reports of illness from nerve agent exposure, since the Skripals and DS Bailey.

Wiltshire Council and other agencies will discuss these latest updates at a public meeting at City Hall on Thursday at 6pm.