Britain and Bulgaria are jointly investigating the 2015 poisoning of a Bulgarian arms dealer to determine whether it involved novichok, the nerve agent used in the poisoning of Russian double-agent Sergei Skripal and his daughter, Yulia, in Salisbury last year.

The investigation was announced on Monday, after the British ambassador to Bulgaria met Boyko Borissov, the country’s prime minister, and other senior officials. It came after reports that one of the potential suspects in the Skripal poisoning was also allegedly in Bulgaria at the time of the poisoning of the arms dealer.

“We are working in a joint team and a close partnership, and we are going to find out the facts in this case,” ambassador Emma Hopkins told reporters in Sofia.

Emilian Gebrev was hospitalised in April 2015 after collapsing at a reception he was hosting in Sofia. His son and one of his company’s executives also fell ill shortly after, and all three were hospitalised. Gebrev fell into a coma, and doctors surmised he had been poisoned, though they could not identify the poison used.

The Bulgarian investigation into Gebrev’s poisoning was reopened in October, after he told Bulgarian prosecutors he believed the substance used against him could have been similar to novichok, Bulgaria’s chief prosecutor Sotir Tsatsarov said on Monday.

The story gained little attention at the time, and reports of a possible novichok link only surfaced in a Bulgarian newspaper report last month. Last week, the investigative site Bellingcat claimed it had discovered that a possible suspect in the Skripal poisoning had been in Bulgaria at the time Gebrev fell ill.

Sergey Fedotov, believed to be an alias for an officer of Russia’s GRU military intelligence service, has not been named by British investigators as a suspect in the Skripal poisoning. However, Russian investigative journalists and Bellingcat believe they have strong evidence that Fedotov works for the GRU, was in the relevant country at the time of both the Gebrev and Skripal poisonings, and used similar travel patterns in both cases, booking return flights but not showing up for them and instead flying using a different route.

Tsatsarov confirmed on Monday that Fedotov had visited Bulgaria three times in 2015 and was in the country when Gebrev was poisoned. “We are establishing all moments while he was on Bulgarian territory, the hotels, the vehicles he used, contacts with Bulgarian citizens,” he said.

British authorities believe Skripal and his daughter were poisoned with novichok, a powerful nerve agent developed by the Soviet Union. They both survived and are currently in an unknown location.

Gebrev also survived, and after he recovered sent urine samples to a Finnish laboratory, which failed to identify the poison but found traces of organophosphates, used in pesticides. No substances from the Chemical Weapons Convention’s banned list were found, Tsatsarov said on Monday.

He said no links had been discovered to the Skripal poisoning but that the investigation was ongoing and added that in December, Bulgaria sent medical data and test results to Britain to check for any possible novichok links. Gebrev has told Bulgarian media that he does not know why he might have been targeted by a foreign intelligence service. The businessman has been involved in arms export for several decades.

On Friday, Vladimir Putin’s spokesman, Dmitry Peskov, said it was difficult to understand why the Bulgarian poisoning allegation had only come to light several years after the event. “Why have we found out about this only now? We do not know whether this corresponds with reality at all,” he said.

Russian authorities have also denied all involvement in the poisoning of the Skripals but the denials have been flimsy. British officers said they were seeking two men who travelled under the aliases Ruslan Boshirov and Alexander Petrov. The two men gave a stilted interview to the Kremlin’s English-language television station, RT, in which they said they were travelling vitamin salesmen visiting Salisbury to view its cathedral spire. They have not surfaced since.

Bellingcat and Russian outlet the Insider claimed the men’s real identities were Anatoliy Chepiga and Alexander Mishkin, and that both worked for the GRU, providing large amounts of circumstantial evidence. Acquaintances in the two men’s home towns also confirmed that the men who appeared on RT were Chepiga and Mishkin.

Additional reporting by Maria Georgieva

• This article was amended on 15 February 2019. Organophosphates are used in pesticides, not fertilisers as an earlier version said.