Three Russians were charged over the poisoning of Emiliyan Gebrev, who fell ill in April 2015 along with his son and company manager.

However, in an open letter Gebrev condemned the fact that the Bulgarian investigation had now been suspended and said that the international focus on Navalny's poisoning "creates an opportunity to widen and deepen its scope".

In the letter, addressed to Prime Minister Boyko Borisov and Chief Prosecutor Ivan Geshev, Gebrev appealed on the Bulgarian government to bring his case "to the agenda of the discussions on these issues, which are beginning in the EU and NATO".

Gebrev told AFP that "shedding light on this case is important for the world... in order to prevent or minimise the possibility of such an event happening again".

The Gebrev case and that of former Russian double agent Sergei Skripal have both been linked, by the investigative Bellingcat website, to a high-ranking Russian military intelligence officer it named as Denis Vyacheslavovich Sergeev.

Bellingcat has reported that Sergeev visited Bulgaria at the same time as Gebrev was poisoned, and that he also entered the UK two days before the Skripal poisoning.

Skripal and his daughter were the victims of a poison attack in the UK in 2018 while Navalny was taken to Germany for medical treatment last month after falling ill in Russia.

Navalny's aides have said they suspect he drank a cup of spiked tea at a Siberian airport.

Germany says there is "unequivocal evidence" Navalny had been poisoned with the Soviet-era nerve agent Novichok, the same substance used in the Skripal attack.

As for the investigation in Bulgaria, it has been "suspended but not closed," Siyka Mileva, spokeswoman for the Bulgarian prosecutors' department, told AFP.

The probe is on hold while prosecutors await replies to requests for help to other countries, Mileva said.