Britain's ambassador Emma Hopkins made the comments after talks with Bulgarian Prime Minister Boyko Borisov and Chief Prosecutor Sotir Tsatsarov.
"We are continuing a close partnership together with a joint team looking into the factual underpinning of these allegations," Hopkins said at a televised news conference.
A report published last week by the investigative website Bellingcat identified a hitherto unknown third suspect in last year's attack in the English city of Salisbury on former Russian spy Sergei Skripal and his daughter.
Both had to be hospitalised after being exposed to the highly toxic nerve agent Novichok in an attack the British government says was "almost certainly" approved by the Russian state.
According to Bellingcat, the third suspect is a Russian military intelligence officer known under the alias "Sergey Fedotov".
The site said Fedotov had arrived in Britain two days before the Skripals were poisoned and was possibly involved in the attack.
Bellingcat has already used open-source techniques to identify two Russian military intelligence officers, Anatoly Chepiga and Alexander Mishkin, accused by Britain of carrying out the attack.
Despite Russian denials that they were involved, both men are now the subject of EU sanctions.
Bellingcat also argues that "Fedotov" may have been involved in the poisoning of Bulgarian arms manufacturer Emiliyan Gebrev, his son and a company executive in 2015.
The prosecutor Tsatsarov on Monday confirmed reports by Bellingcat and in Bulgarian media that "Fedotov" had travelled to Bulgaria three times in 2015; in February, again in April -- shortly before Gebrev's poisoning -- and finally in May.
Tsatsarov said that the 2015 poisoning was "most probably" carried out using the insecticide Chlorpyrifos, traces of which were found on the coffee machine in Gebrev's home.
Gebrev collapsed at a reception he was hosting in Sofia on 28 April 2015 and fell into a coma. His son and one of his company executives were also treated for poisoning symptoms although all three recovered.
Tsatsarov said that the probe into the Gebrev poisoning had been reopened in October 2018, and that UK authorities had "been provided with full access to all the documents and materials on this case".
Bulgarian media reports said Gebrev could have been targeted by Russia for exporting arms to Ukraine.
The Salisbury attack, the first offensive use of chemical weapons in Europe since World War II, caused an international outcry and prompted a mass expulsion of Russian diplomats by Western nations -- but not by Bulgaria.
While Skripal and his daughter survived the attack, a British woman died last June after her partner picked up a discarded perfume bottle that investigators believe was used to carry the Novichok.