British police believe a Russian ex-spy and his daughter first came into contact with a military-grade nerve agent at their front door.

Deputy Assistant Commissioner Dean Haydon says in a statement police are now focusing their investigation in and around Sergei Skripal’s home.

London’s Metropolitan Police say they have “identified the highest concentration of the nerve agent, to-date, as being on the front door of the address”.

Mr Haydon added traces of the nerve agent, dubbed Novichok, was also found at some of the other scenes detectives have been working at over the past few weeks, but at lower concentrations.

Sergei and Yulia Skripal remain in critical condition after they were poisoned in a nerve agent attack in the southwestern UK city of Salisbury on March 4.

The revelation is significant because it’s the first time police have offered any suggestion about where the Skripals were poisoned.

Police have also searched a variety of sites around Salisbury, including a pub, a restaurant and a cemetery.

Around 250 counter-terrorism detectives are currently working “around the clock” on the case, which could continue for months, police say.

“Those living in the Skripals’ neighbourhood can expect to see officers carrying out searches as part of this but I want to reassure them that the risk remains low and our searches are precautionary,” Mr Haydon said.

“I’d also like to thank the local community for their continued support and understanding. The unique circumstances of this investigation means that officers are likely be in the area for several weeks and months.”

Britain blames Russia for the poisoning, triggering a diplomatic dispute between Moscow and Western countries. Moscow vehemently denies any involvement.

Police have gathered more than 5000 hours of CCTV and examined over 1350 exhibits that have been seized. Around 500 witnesses have been identified and hundreds of statements taken, according to reports.