The residue of the nerve agent used in the attempted murder of the Skripals in Salisbury could still be as potent as it was when it was deployed more than six weeks ago, government scientists believe.

Experts responsible for the decontamination effort in the cathedral city said there could still be “hotspots” yet to be discovered where the novichok agent remains in high concentrations.

The revelations were made at a public meeting held in Salisbury as new barriers were put in place around key sites connected to the attack on the former Russian spy Sergei Skripal and his daughter, Yulia, signalling the start of the lengthy clean-up process. Almost 200 military personnel in protective suits and boots will spend months decontaminating nine sites.

Officials from the Department for Environment, Food and Rural Affairs (Defra) came under fire during the meeting from some members of the public who suggested that the decontamination process was going to take too long.

Ian Boyd, the chief scientific adviser at Defra, told one critic: “You are underplaying the toxicity of this chemical. You’re also underplaying how the chemical has been spread.”

He compared the spread of the chemical to ink. “You dip your finger in ink and if that ink doesn’t dry and you go through your normal daily activities, you will find that ink in a lot of different places.

“Ink is okay – you can see it. We’re dealing with a chemical that you cannot see. When you put that into a system of trying to make sure you decontaminate 100% from every site, you have to be very, very careful. It does not degrade as fast as you think it does. The chemical does not degrade quickly. You can assume that it is not much different now from the day it was distributed.

“This chemical can degrade in the environment but, under [some] situations, it will degrade much more slowly. We have to make the assumption that, in certain circumstances, there will be relatively high concentrations, probably at very specific locations which could be at levels that are toxic to individuals. We do know there are hotspots like that. We have to make the assumption that there are still hotspots to find.”

He emphasised that the hotspots would be within locations already known about and under guard – such as the park where the Skripals fell ill and the pub and restaurant where they ate and drank beforehand.

Explaining the decontamination process, he said sites would be tested and contaminated items removed. Cleaning would then take place and the sites would be retested. A committee of experts would ultimately say whether the sites were clear. Bespoke plans had been put together for each site. “The deeper you look into this, the more complicated it is,” Boyd said.

Tracy Daszkiewicz, Wiltshire council’s director of public health, said no further cases of illness had been reported since the beginning of March, when the attack took place.

Police and council officials will move out of offices at Bourne Hill in the city on Friday for eight weeks so that they can be decontaminated and deep-cleaned. A police property store where evidence was taken to shortly after the attack also needs decontamination, along with ambulance stations at Salisbury and Amesbury and the home of DS Nick Bailey, the officer who fell ill after responding to the attack.

Attention will then focus on the Maltings park where the Skripals fell ill, and the Mill pub and Zizzi restaurant, which the Skripals visited before they were found.

Investigative rather than clean-up work is to continue to take place at Skripal’s home on the outskirts of Salisbury for some weeks before the clean-up can begin there.

Earlier this week, Defra said the nerve agent used in the attack was delivered in liquid form. Police had previously said they believed that the pair had been poisoned at the front door of Skripal’s home. Specialists found the highest concentration of the nerve agent on the door.

Last week, previously classified intelligence about the attack claimed that Russia had tested whether door handles could be used to deliver nerve agents.