Former Russian spy Sergei Skripal and his daughter Yulia were poisoned at their home, police believe.

As a result of detailed forensic and scientific examination, detectives specialists identified the highest concentration of the nerve agent Novichok on their front door.

Around 250 counter terrorism detectives continue to work around the clock on the investigation, supported by a range of experts and partners.

Officers are continuing to trawl through more than 5000 hours of CCTV and examine over 1350 exhibits that have been seized.

Around 500 witnesses have been identified and hundreds of statements taken.

Deputy Assistant Commissioner Dean Haydon, Senior National Coordinator for Counter Terrorism Policing said: "At this point in our investigation, we believe the Skripals first came into contact with the nerve agent from their front door.

"We are therefore focusing much of our efforts in and around their address. 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."

The attack on the Skripal and his daughter has brought Anglo-Russian diplomatic relations to one its lowest points in years.

In the wake of the incident in Salisbury, the British government expelled 23 Russian diplomats. In retaliation, the Kremlin announced it would be sending home 23 British officials home.

Foreign Secretary Boris Johnson accused Russia of trying to conceal "the needle of truth in a haystack of lies" over the poisoning.

More than 20 countries have now expelled Russian diplomats over the Skripal poisoning despite the Kremlin continuing to deny any involvement.

Police were called by members of the public on March 4 after Mr Skripal, 66, and 33-year-old daughter, Yulia, were spotted slumped on a bench near the Maltings shopping centre.

The pair remain unconscious and in critical but stable conditions.