Bulgaria and the UK say they will jointly investigate possible connections between the poisoning of Russian former spy Sergei Skripal in Salisbury and the poisoning of the Bulgarian owner of an arms factory in 2015.
Britain's ambassador to Sofia, Emma Hopkins, said after a meeting with Bulgarian Prime Minister Boyko Borissov and law enforcement officials that information about the possible poisoning of Emilian Gebrev had been discussed.
"We are working in a joint team and a close partnership, and we are going to find out the facts in this case," Ms Hopkins told reporters.
"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," she said.
Bulgarian chief prosecutor Sotir Tsatsarov said that a Russian citizen, Sergei Fedotov, made three trips to Bulgaria in 2015, one of them in April, when Mr Gebrev was poisoned. Mr Gebrev survived.
According to investigative group Bellingcat, Fedotov was also suspected of being involved in the Novichok nerve agent poisoning of Mr Skripal and his daughter, Yulia. He arrived in Britain two days before the March 2018 attack, on the same day as the two suspected attackers.
Mr Tsatsarov said prosecutors had compiled "significant" data on Fedotov's trips to Bulgaria as part of an investigation into the poisoning that was reopened in October.
"We are establishing all moments while he was on Bulgarian territory, the hotels, the vehicles he used, contacts with Bulgarian citizens," Mr Tsatsarov said.
Mr Tsatsarov said Mr Gebrev had written to him in October saying that he had reason to suspect he might have been poisoned by a substance from the same family as Novichok, the nerve agent used in Salisbury.
He acknowledged there was no scientific evidence, however. Mr Gebrev had requested and paid for blood and urine tests at Finland's Verifin institute. They confirmed the presence of organophosphorus compounds, which are used in some pesticides, but the exact pesticide was not identified.
The tested samples were not found to contain any substances figuring on the list of the Chemical Weapons Convention.
The use of Novichok in Salisbury has been confirmed by laboratories. Both Skripals survived after weeks in the hospital, and after their release they were taken to an undisclosed location for their safety.
UK officials have blamed the attack on the Russian military intelligence agency GRU and charged two Russian suspects. The men travelled under the names Alexander Petrov and Ruslan Boshirov.
Russian authorities have denied involvement and Moscow refused to extradite the men to the UK.
- Press Association