The nerve agent used to poison the former double agent Sergei Skripal and his daughter Yulia in Salisbury was delivered “in a liquid form”, the Department for Environment (Defra) has revealed for the first time.

At a briefing on Tuesday, Defra revealed that a “very small amount” of the nerve agent was used in the attack, and that the highest concentration was found at Skripal’s home. 

Clean-up work is set to begin at nine sites across the city, Defra said. A tenth site, an area of the London Road cemetery which was found not to be contaminated will reopen to the public.

“We either know there is contamination there (in the nine sites) or we think there is a probability we could find contamination there,” Defra said.

Salisbury residents have been warned to expect an increase in activity as investigators wearing protective clothes remove items and chemically clean the areas.

Skripal, 66, a former double agent, and his daughter Yulia, 33, were poisoned in the town on March 4 with a Soviet-developed nerve agent called Novichok.

Yulia was released from Salisbury District Hospital last week and reportedly taken to a secret location, but her father is still being treated. Doctors have said he is recovering rapidly, and would be released in “due course”. 

The government has said that its analysis by military experts at Porton Down showed the Skripals were affected by Novichok – a military grade nerve agent developed by Russia.

The international chemical weapons watchdog has supported this assessment.

Russia has continued to deny it was involved on the attack.  

The update from Defra came as Jeremy Corbyn again refused to blame Moscow, saying he wants to see “incontrovertible evidence” first.

Corbyn, who has been given intelligence briefings on the incident, told BBC One’s The Andrew Marr Show he wanted further proof.

“I want to see incontrovertible evidence of it,” he said.

Asked if he agreed with his shadow chancellor John McDonnell that it was a Russian state-sponsored attack, Corbyn replied: “If we are going to make a very, very clear assertion like that we have got to have the absolute evidence to do it.”

The Russian Embassy, meanwhile, has published an 8,000-word report into the poisoning row, setting out a timeline of events, the British response and the lack of information it claims to have received following official requests.

The report reiterates the Kremlin’s claim Russia had “nothing to do” with the Salisbury attack and claimed the UK Government had classified information in a similar way to the Alexander Litvinenko case.

The report said: “The same script, but in a fast-forward mode, is being played this time.”