Nerve agent found on Skripal's front door

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."

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.

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