The UK and Bulgaria are investigating possible connections between the Salisbury nerve agent attack and a 2015 poisoning in Bulgaria.
One of the three Russian suspects linked to the poisoning of ex-Russian spy Sergei Skripal is alleged to have been involved in the poisoning of the Bulgarian owner of an arms factory three years earlier.
Russian citizen Sergei Fedotov made three trips to Bulgaria in 2015 – including once in April, which is when Emilian Gebrev was poisoned, Bulgarian chief prosecutor Sotir Tsatsarov said.
Mr Gebrev survived the attack, but a Finnish laboratory has been unable to identify the poison that was used.
Britain’s ambassador to Sofia, Emma Hopkins, said she had discussed the possible poisoning of Mr Gebrev in a meeting with Bulgarian Prime Minister Boyko Borrissov.
She told reporters on Monday: “We are working in a joint team and a close partnership, and we are going to find out the facts in this case.
“All questions about the national security of the UK and Bulgaria are of paramount importance to us, and we will continue this investigation even after Brexit.”
Investigative group Bellingcat said Fedotov was also suspected of being involved in the novichok nerve agent poisoning of Mr Skripal and his daughter Yulia in Salisbury last year.
Fedotov is said to have arrived in the UK two days before the March 2018 attack, on the same day as the two suspected attackers.
“Significant” data had been gathered on Fedotov’s trips to Bulgaria as part of an investigation into the poisoning, Mr Tsatsarov added.
He said: “We are establishing all moments while he was on Bulgarian territory, the hotels, the vehicles he used, contacts with Bulgarian citizens.
“Since then, we have been working in full co-operation and co-ordination with the British services.
“They have full access to all documents and all the materials in the case and the results of all investigative actions.”
Mr Tsatsarov said that Mr Gebrev had written to him in October to say he had reason to suspect he may have been poisoned by a substance from the same family as novichok.
However, he acknowledged there was no scientific evidence to back his claims.
Blood and urine tests confirmed the presence of organophosphorus compounds in Mr Gebrev’s system, which are used in some pesticides.
Mr Gebrev’s condition improved and he was discharged from hospital a month later.
Laboratories previously confirmed that novichok was used in the Salisbury poisonings, with the Skripals surviving after weeks in hospital.
UK officials have blamed the attack on the Russian military agency GRU and charged two Russian suspects, who went by the names Alexander Petrov and Ruslan Boshirov.
Russian authorities have denied involvement and Moscow refused to extradite the men to the UK.