The nerve agent used to poison Sergei Skripal and his daughter, Yulia, in Salisbury was delivered in a liquid form, the Department for Environment, Food and Rural Affairs has said.

A very small amount of the novichok nerve agent was used in the attack and the highest concentration was found at Skripal’s house, it emerged in a press briefing in the city.

Work to decontaminate sites in Salisbury affected by the nerve agent is beginning, but will take months and involve almost 200 members of the armed forces.

Nine sites had been identified as needing some sort of specialist cleaning, Defra said.

They include part of the Maltings shopping centre where the former Russian spy Skripal and his daughter collapsed, the Zizzi restaurant and Mill pub which they visited that day, and the family home on the outskirts of the city. The home of the police officer injured in the incident, DS Nick Bailey, also needs decontaminating.

Police previously said that they believed 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, police said.

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

A small cordoned area of the cemetery where the remains of Skripal’s wife and son lie has been reopened after “extensive investigations and testing” established that it was not contaminated.

The other nine potentially contaminated sites will remain secured. Work to clean each site will involve a process of testing, removal of items which may have been contaminated, chemical cleaning and retesting. Sites will not be released back into use until test results and the work undertaken has been reviewed and approved by the government’s decontamination science assurance group.

Quick Guide What is novichok? Show Novichok refers to a group of nerve agents developed by the Soviet Union in the 1970s and 80s to elude international restrictions on chemical weapons. Like other nerve agents, they are organophosphate compounds, but the chemicals used to make them, and their final structures, are considered classified in the UK, the US and other countries. The most potent of the novichok substances are considered to be more lethal than VX, the most deadly of the familiar nerve agents, which include sarin, tabun and soman. Novichok agents work in a similar way, by massively over-stimulating muscles and glands. Treatment for novichok exposure would be the same as for other nerve agents, namely with atropine, diazepam and potentially drugs called oximes. The chemical structures of novichok agents were made public in 2008 by Vil Mirzayanov, a former Russian scientist living in the US, but the structures have never been publicly confirmed. It is thought they can be made in different forms, including as a dust aerosol. The novichoks are known as binary agents because they only become lethal after mixing two otherwise harmless components. According to Mirzayanov, they are 10 to 100 times more toxic than conventional nerve agents. Photograph: Matt Cardy/Getty Images Europe

Defra is planning and overseeing the work based on expert advice from the Defence Science and Technology Laboratory (Dstl) at Porton Down, Public Health England, the Department for Health and Social Care, the Home Office and the Ministry of Defence.

The clean-up operation will be carried out in partnership with Wiltshire council with support from the MoD, which is providing specialist teams to carry out work on the sites. Around 190 specialist personnel from the army and RAF will support the operation.

Defra’s chief scientific adviser, Prof Ian Boyd, the chair of the decontamination science assurance group, said: “Our approach is based on the best scientific evidence and advice to ensure decontamination is carried out in a thorough and careful way. Our number one priority is making these sites safe for the public, so they can be returned to use for the people of Salisbury.

“Thanks to detailed information gathered during the police’s investigation, and our scientific understanding of how the agent works and is spread, we have been able to categorise the likely level of contamination at each site and are drawing up tailored plans.

“Meticulous work is required and we expect it will be a number of months before all sites are fully reopened.”

Defra said the public would begin to see more activity in the city as the work began. In the coming days residents can expect to see cordons around the most public sites replaced with secure fencing, backed by police patrols and security guards.

At certain points during decontamination, some cordons will be temporarily expanded to allow workers access to the sites with specialist equipment and ensure public safety as work is carried out.

The leader of Wiltshire council, Lady Jane Scott, said: “We are pleased that work will be starting to decontaminate the sites affected by the shocking attack in our city. Working together with local and national agencies we are doing all we can to help Salisbury return to normal.

“Our main concern is to ensure that Salisbury is safe for residents, businesses and visitors and that the city can focus on the future, its recovery and that it will go from strength to strength.”

Defra said the current scientific assessment was that the remainder of Salisbury was safe for residents and visitors. Public Health England has reaffirmed that the risk to the general public is low.

Other sites that need decontamination include the compound where Skripal’s car was stored and two ambulance stations.