A third man has been named as a suspect in the poisoning of Sergei and Yulia Skripal in Salisbury last year.
Investigative website Bellingcat claims that he is Denis Vyacheslavovich Sergeev, a Russian military intelligence officer.
It says he came to the UK at the same time as two suspects alleged to have carried out the March 2018 attack.
The website claims the officer travelled internationally under the pseudonym Sergey Fedotov.
MI6 double agent Sergei Skripal, 67, and his daughter Yulia, then 33, were poisoned with a nerve agent known as novichok in Salisbury. Both of them survived.
Prior to the latest claim, two Russian nationals were named as suspects.
Anatoliy Vladimirovich Chepiga and Alexander Mishkin have been linked to the Russian military intelligence agency GRU.
The Kremlin has not commented on the latest report but it has previously expressed doubts over Bellingcat's reports about Mr Chepiga and Mr Mishkin.
The website says that Mr Fedotov was booked onto a flight which left the UK last March but that he missed it.
It says he travelled instead to Rome, from where he went to Moscow.
It previously reported that he used a similar travel pattern in 2015 in Bulgaria - missing a booked flight and returning to Moscow from Istanbul.
During that trip, Bellingcat says a Bulgarian arms trader, Emilian Gebrav, and his son needed hospital treatment after contact with an unidentified poison. Mr Gebrav survived.
The website says that Mr Sergeev travelled with one of the other two suspects on at least one occasion, and made multiple trips to the UK.
British officials are understood to be investigating the Bulgarian reports.
A Kremlin spokesman previously said Russia did not know "whether this is true at all".
Responding to the website's initial investigation, a Kremlin spokesman told the BBC: "We don't know how far this corresponds with reality, whether it's real at all.
"We don't know what the report's authors based their work on - how competent they are - who they are - and whether this is true at all."
The British government blamed the Salisbury attack on the GRU.
Prime Minister Theresa May said the attack on Mr Skripal had "almost certainly" been approved by the Russian state.
Moscow has consistently denied any involvement in the Salisbury poisonings.
The Metropolitan Police say they are continuing to pursue a number of lines of enquiry including identifying any other suspects who may have been involved in carrying out or planning the attack.