Detective became ill after going to Skripal's house

A Russian hit squad poisoned Sergei Skripal and his daughter by smearing nerve agent on his front door.

Counter-terrorism police revealed for the first time that they believed Skripal and his daughter Yulia "first came into contact" with Novichok at their home.

The "highest concentration of the nerve agent" was discovered on the front door of the house in a cul-de-sac close to Salisbury city centre, Scotland Yard disclosed.

Detective Sergeant Nick Bailey, who was also admitted to hospital, is thought to have become ill after going to the house and was one of the first on the scene.

The disclosure will intensify the manhunt for the would-be assassins who targeted Skripal, 66, and his 33-year-old daughter.

It will also heap further pressure on the Kremlin, which is accused of being behind the attack, for putting at risk anybody who had visited the house.

Assistant Commissioner Dean Haydon, the 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 Skripals remain in hospital in critical conditions.

Skripal, a former Russian double agent, was sent to Britain in a spy swap in 2010.

The Met said 250 counter terrorism detectives continued to work "around the clock", supported by experts and intelligence officers.

Police have more than 5000 hours of CCTV footage and more than 1350 exhibits have been seized.

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

Traces of the nerve agent were also at other sites in Salisbury, including a pizza restaurant and pub the Skripals visited before collapsing on March 4. These were "at lower concentrations" to that found at the house.

Specialist search teams are starting to finish their inquiries and are handing control of sites back to Wiltshire Police, including the cemetery where Skripals' wife and son are buried and the Maltings shopping precinct close to where they collapsed.