Bulgaria has opened an investigation following reports that a third suspect in Salisbury attack was also involved in a 2015 poisoning in Bulgaria. Tsvetan Tsvetanov,a senior official in the ruling GERB party, said intelligence officers would present their evidence on the topic on Thursday at a parliamentary security committee.

"I am certain that the necessary coordination has already been set up between the Bulgarian, British and European authorities on the case and they are working actively on it," Mr Tsvetanov said.

It comes after investigative group Bellingcat reported an alleged Russian military intelligence agent arrived in Bulgaria in April 2015, a few days before Bulgarian businessman Emilian Gebrev was poisoned by an unidentified substance.

Mr Gebrev, an arms industry executive, survived but authorities still don't know who poisoned him. Bellingcat said on its website the 45-year-old Russian agent traveled under the alias Sergei Vyacheslavovich Fedotov and had been "conclusively identified as an agent of Russian military intelligence" for Moscow's GRU agency. Bellingcat said Fedotov also was suspected of being involved in the Novichok nerve-agent poisoning of Russian ex-spy Sergei Skripal and his daughter, Yulia, in the English city of Salisbury.

He arrived in Britain two days before the March 2018 attack. Both Skripals survived after weeks in the hospital and after their release were taken to an undisclosed location for their safety, the British government has said. British officials have blamed the attack on the GRU and charged two Russian suspects. The men traveled under the names Alexander Petrov and Ruslan Boshirov.