The nerve agent used to poison Sergei Skripal and his daughter Yulia in Salisbury was delivered in a liquid form, the government has said.

Nine sites in the Wiltshire city were potentially contaminated following the 4 March attack, carried out with a "very small amount" of a military-grade novichok, an official from the Department for Environment, Food and Rural Affairs (Defra) revealed.

The highest concentration was found at the Russian former spy's house, he told a press briefing in Salisbury.

“In this instance direct contact is required for a person to be poisoned - only a small proportion of the material is transferred and the substance is diluted in each secondary and tertiary contact,” the official added.

“The class of nerve agent does not produce significant vapour or gas and can only move between sites by direct transfer by a contaminated person or item.”

The official confirmed that novichok “doesn’t just disappear”, meaning that if the substance was detected in the ongoing police investigation it will still be present.

Contaminated parts of the city will be sealed off behind high security barriers for months as work continues to make the city safe.

Skripal attack aftermath – in pictures Show all 15 1 /15 Skripal attack aftermath – in pictures Skripal attack aftermath – in pictures Scene of attack Members of the emergency services in hazard suits fix the tent over the bench where Sergei and Yulia Skripal were found unconscious on a park bench in Salisbury in March 2018. Getty Skripal attack aftermath – in pictures Victim - Sergei Skripal The retired Russian colonel and former double agent for MI6 was in a critical condition in hospital for more than two months after being exposed to novichok in Salisbury. He was given refuge in the UK after being jailed in Moscow for treason. Mr Skripal came to Britain as part of a high-profile “spy swap” in 2010 in which four men were exchanged for ten Russian "sleeper agents" in the US. In this image he is speaking to his lawyer from behind bars in Moscow in 2006. AP Skripal attack aftermath – in pictures Victim - Yulia Skripal Yulia Skripal was struck down by a novichok poison alongside her father Sergei. Facebook Skripal attack aftermath – in pictures Scene of attack A police officer stands guard outside a branch of the Italian chain restaurant Zizzi where the pair dined at before falling ill. It was boarded off whilst investigators worked on the building and later found traces of the chemical weapon within it. AFP/Getty Skripal attack aftermath – in pictures Scene of attack Large areas of central Salisbury were cordoned off by police following the discovery of the Skripals. Traces of nerve agent were also found in The Mill pub. PA Skripal attack aftermath – in pictures Victim - Nick Bailey Detective Sergeant Nick Bailey, rushed to the aid of the Russian ex-spy and his daughter who were targeted with a nerve agent. He was hospitalized after aiding them and didn't leave until three weeks after the attack. Wiltshire Police/Rex Skripal attack aftermath – in pictures Police investigation - Skripal’s home Police believe they were poisoned at home, and detectives found the highest concentration of novichok on the front door of Mr Skripal’s house. Getty Skripal attack aftermath – in pictures Theresa May visits scene of attack Britain's Prime Minister Theresa May spokes with Wiltshire Police's Chief Constable Kier Pritchard near where the Skripal's were found. Britain expelled 23 Russian diplomats over the nerve agent poisoning and suspended high-level contacts, including for the World Cup on March 14. Theresa May told parliament that Russia had failed to respond to her demand for an explanation on how a Soviet-designed chemical, Novichok, was used in Salisbury. AFP/Getty Skripal attack aftermath – in pictures Skripal days before attack Sergei Skripal days before he was exposed to Novichok, that has left him fighting for life. ITV News Skripal attack aftermath – in pictures Police investigation - military involvement British soldiers were deployed soon after the attack to help a counter-terrorism investigation into the nerve agent attack. One of the places they were asked to help out with was Skripal's home and it's surrounding. They were asked to remove a vehicle connected to the agent attack in Salisbury, from a residential street in Gillingham. AFP/Getty Skripal attack aftermath – in pictures Police investigation Personnel in protective coveralls and breathing equipment cover an ambulance with a tarpaulin at the Salisbury District Hospital. AFP/Getty Skripal attack aftermath – in pictures Police investigation The investigation extended to the grave of Sergei Skripal's son Alexander in London Road cemetery. Getty Skripal attack aftermath – in pictures Police investigation The Counter Terrorism Policing Network requested assistance from the military to remove a number of vehicles and objects from Salisbury. EPA Skripal attack aftermath – in pictures Home Secretary visits scene of attack Home Secretary Amber Rudd visited the scene of the nerve agent attack at the Maltings shopping centre on 9 March. Getty Skripal attack aftermath – in pictures Yulia Skripal speaks for the first time Yulia Skripal, speaking for the first time, said she felt lucky to have survived the nerve agent attack in Salisbury which left her fighting for life. Ms Skripal said her life had been “turned upside down” by the assassination attempt. But the Russian national added she hoped to return to her homeland one day, despite the Kremlin being blamed for the attack. Reuters

The “primary site of contamination” is Mr Skripal’s house and eight other sites are believed to have been contaminated, including the Zizzi restaurant and The Mill pub the former double agent visited with his daughter, police and ambulance stations and Detective Sergeant Nick Bailey’s home.

Public Health England (PHE) reassured members of the public that the risk to health remained low and said sites would be chemically cleaned and retested before being reopened.

The cemetery where the remains of Mr Skripal's wife and son are buried was found not to be contaminated and has reopened to the public.

Police have said previously that the highest concentration of the nerve agent was found on the former double agent's front door.

The 66-year-old and his daughter, 33, were left fighting for life following the suspected assassination attempt and were initially feared to have little chance of survival. But their conditions have improved and Ms Skripal has since been discharged from hospital.

Her father remains in hospital in a stable condition.

Russia claim trace of Western-made nerve agent seen in UK samples

The Maltings shopping centre and a Zizzi restaurant where the Skripals dined on the day they fell ill are among the nine locations identified as requiring "some level of specialist cleaning", said Defra. Three of the contaminated sites are in the city centre.

Public Health England stressed the risk to the public was low.

"All remaining potentially contaminated sites will remain secured and the current scientific assessment is that the remainder of Salisbury is safe for residents and visitors," said Defra.

Twenty-one people received medical treatment for exposure to the novichok in the days following the attack.

Several were police officers, including DS Bailey, who fell seriously ill and was treated in intensive care before being discharged two weeks later.

Four independent laboratories have concluded the chemical used in the attack was a novichok-class nerve agent, which were originally developed by the Soviet Union, the Organisation for the Prohibition of Chemical Weapons said last week.

Their conclusions match that of UK scientists at the Porton Down defence laboratory, which the government said proved Moscow was culpable for the attack after allegedly spying on the Skripals for at least five years.

The Foreign Office said there was ”no alternative explanation about who was responsible – only Russia has the means, motive and record”, and demanded the Kremlin “give answers”.