Sergei and Yulia Skripal first came into contact with novichok at their home, detectives believe.

Specialists have identified the highest concentration of the nerve agent on their front door.

Detectives will continue to focus their enquiries around the address in Salisbury for the coming weeks - and possibly months - as the investigation continues.

In a statement, the Metropolitan Police said officers would conduct thorough searches in the areas surrounding the former spy's semi-detached property to gather evidence and as a precautionary measure.

Dean Haydon, the senior national co-ordinator 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."

Image: Sergei Skripal and Yulia Skripal were poisoned

Traces of the nerve agent have been found at some of the other crime scenes that detectives have been examining over the past few weeks, but at lower concentrations.

A dozen policemen, who were not wearing protective overalls, searched an alley beside the victim's house on Wednesday for what is thought to be the first time.

The investigation into who poisoned Mr Skripal, 66, and his daughter, 33, is one of the largest and most complex investigations undertaken by British counter-terrorism police.

The pair were found collapsed on a bench on Sunday 4 March in a critical state.

A Russian law enforcement committee which is running an investigation into the poisoning said it has sent a request to Britain to provide Moscow with legal assistance.

Moscow is seeking "the British colleagues to perform a number of procedural actions aiming at establishing the circumstances of the crime, as well as provide copies of the materials of the criminal investigation".

Russia is asking London to provide it "with the results of the inspection of the place where Yulia Skripal were found unconsciousness as well as with the results of her medical examination".

Meanwhile, New Zealand has said it will not allow entry to Russian diplomats expelled by other countries in response to the nerve agent attack.

About 250 counter-terrorism detectives are working around the clock on the investigation, according to the Met Police.

Salisbury nerve agent bench removed

More than 5,000 hours of CCTV is being examined and more than 1,350 pieces of evidence are being scrutinised.

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

On Wednesday, the 24th day of the investigation, Mr Haydon thanked the local community "for their continued support and understanding".

Specialist search officers are concluding their work at some of the locations involved in the investigation and turning the areas over to Wiltshire Police.

These areas include the London Road Cemetery, where investigators in hazmat suits were seen early on in the case.

The UK has accused Russia being responsible for the attack, resulting in more than 150 Russian diplomats being expelled from countries across the globe.

Russia rejects the accusation and has challenged the UK to prove the British intelligence services did not conduct the assassination attempt.

The best friend of Mr Skripal told Sky News he believes the poisoned double agent and his daughter are so critically ill they should be allowed to die.