A deadly nerve agent was applied to former spy Sergei Skripal’s front door while he and his daughter, Yulia, were at home, a counter-terrorism chief has revealed.

The Skripals must have been just metres away when the door handle of his home in Salisbury was daubed with novichok, an inquiry into the 2018 poisonings has heard.

Sitting for the first time in London, the inquiry also heard that Skripal, who was settled in the Wiltshire city after a spy swap, told friends that Vladimir Putin would “get him” if he returned to Russia.

The Skripals’ friend Ross Cassidy described how he got to know the ex-spy when he moved in next door in 2010.

He said: “I noticed a new family. I pretended to tend to my fish pond. Sergei leaned over the fence and said ‘hello’. We had a good old chat. I invited them for a drink and that opened the door to friendship.”

Cassidy, a former member of the Royal Navy, said they enjoyed barbecues, drinks in a railway social club and a New Year’s Eve party together. “Sergei amused me in an ironic sort of way. Our families knew each other, our friends became Sergei’s friends,” he said.

Skripal told Cassidy that he had served in the military and had become a diplomat. But Cassidy said: “Sergei did say he could not go back to Russia or there would be reprisals. He would not go into much detail what this was all about but he did say he knew Putin personally and said Putin would get him.”

Cassidy said his wife, Maureen, Googled Skripal. “We could see he was involved in spying on Russia. We never told Sergei we knew this information.”

On 3 March 2018, the day before the poisonings, the Cassidys and Sergei Skripal went to pick up Yulia, who lived in Russia, from Heathrow. When they got back to Salisbury, Maureen Cassidy popped into the Skripal house and used the bathroom. Skripal touched the door handle when he opened it and Maureen Cassidy believes she may have touched it as she left.

Cmdr Dominic Murphy, the head of the Metropolitan police’s counter-terrorism command, said the Skripals did not leave the house again that night or the next morning.

He said a vital tactic in investigations into chemical and biological attacks was to establish the “clean/dirty” line – the point at which contamination occurs. Because the Skripals did not fall ill that night and there was no evidence that Maureen Cassidy or the Cassidys’ vehicle was contaminated with novichok, the police concluded that the poison had not been applied by 6pm.

The Skripals were at home all morning on 4 March before leaving to go to lunch in Salisbury at about 1.30pm. They fell ill in the city centre later that afternoon.

Murphy said: “I set the time parameters of when the novichok was likely to have been applied to that door as 18.00 on the Saturday and 13.30 on the Sunday. That allowed us to focus on that slightly smaller period.”

It also emerged that the Skripals were both contaminated because both touched the door handle as they left the house. Sergei left first and sat waiting in his car while Yulia finished doing her hair.

Murphy said it was a “significant milestone” when police understood why both were poisoned. He said the door handle was considered “ground zero”.

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While in Salisbury on the afternoon of 4 March, Sergei Skripal, who by this time was contaminated with novichok, handed a piece of bread to a boy so he could feed ducks.

Murphy said the boy and two friends he was with had been ill for a day or two afterwards but the illness passed, and when they were tested several weeks later, no trace of contamination was found.

A theory put forward by Ross Cassidy that they were followed as they returned from Heathrow to Salisbury was discounted by Murphy. He said a BMW spotted by Cassidy apparently tailing them was a police car on an unrelated job.

Murphy was asked about a dossier the Russian embassy has issued on the “unanswered questions” around the poisoning. It claimed that Sergei Skripal’s car was seen out and about on the morning of 4 March and his and Yulia’s mobiles were turned off for four hours at that time.

He said the claims were untrue and that there were many hundreds, if not thousands, of red herrings in such investigations. He said there was “nothing to suggest” the Skripals had not been at home when the novichok was applied to the door handle.

The embassy also said it was “inconceivable” that there was no CCTV cameras at Sergei Skripal’s house. The hearing was told he did not want security measures because he did not want to make his home conspicuous or “live under surveillance”.

The inquiry is examining how Dawn Sturgess, 44, died in July 2018 after she was exposed to novichok, which was apparently left in a discarded perfume bottle.

The inquiry continues.