The man, from Russia’s military intelligence service the GRU, is understood to have visited Salisbury ahead of the poisoning of Sergei Skripal and his daughter Yulia, according to The Daily Telegraph.
The paper said the suspect was likely to have fed back details including the layout of the cul-de-sac where Mr Skripal lived and which door he used to enter and leave.
A Scotland Yard spokeswoman said: “We will not be discussing any further details in what remains a live investigation.”
UK authorities believe two Russians, using the aliases Alexander Petrov and Ruslan Boshirov, smeared the highly toxic Novichok chemical on a door handle at the Wiltshire home of former GRU officer Mr Skripal on March 4.
The attack left Mr Skripal and his daughter critically ill, and Dawn Sturgess, 44, who was later exposed to the same nerve agent, died in July.
On Wednesday, the investigative group Bellingcat identified one of the suspects as Colonel Anatoliy Chepiga - a highly decorated GRU officer.
Bellingcat said it had identified Chepiga, who travelled to the UK with a passport issued in the assumed name of Ruslan Boshirov, by trawling through online records from Russian military academies.
The Government has declined to comment officially on the report, although it has previously said the two suspects wanted for the attack on the Skripals were GRU officers.
Bellingcat is reportedly close to identifying the second suspect.
A spokeswoman for the Russian Foreign Ministry dismissed Bellingcat’s claims, and said they were part of an “information campaign” to distract from the investigation into what really happened in Salisbury.
Meanwhile, the Government dismissed allegations that Russian security services infiltrated the British Embassy in Moscow to obtain visas for the suspected poisoners.
A Home Office spokeswoman said: “We reject these claims entirely.
“Applications of this type would be decided by a visa officer in the UK and not in the British Embassy in Moscow.”