Two Russian diplomats accused of espionage by Bulgarian prosecutors have been ordered to leave the country, Sofia announced on Monday.
The Bulgarian Minister of Foreign Affairs said in a statement that it has informed the Russian Embassy that two of its staff have been declared "persona non grata". They have been given 72 hours to leave the country.
"The Ministry of Foreign Affairs received a letter from the Prosecutor's Office of the Republic of Bulgaria stating that the two diplomats had carried out activities in the country that are incompatible with the Vienna Convention on Diplomatic Relations," the statement said.
On Saturday, Bulgarian authorities said they had arrested and charged six people among serving and retired military officials who allegedly spied for Russia.
The Prosecutor General’s spokeswoman, Siika Mileva, told reporters on Friday that "several acting and retired members of the Bulgarian armed forces have been detained on suspicion of passing classified information to a foreign state."
"It’s the first time that an espionage group has been broken in Bulgaria," she said. "Their criminal activity endangers our national security."
This prompted Prime Minister Boyko Borissov to call on Russia "to stop spying in Bulgaria".
Washington and London have both expressed support for Bulgaria with Ned Price, the spokesman for the US State Department, writing on Twitter on Saturday that the country "strongly supports Bulgaria's sovereignty and stands with Bulgarians against these malign activities on their territory."
Britain's Foreign Secretary, Dominic Raab, wrote that the UK "fully supports Bulgaria's efforts in disrupting an alleged spy ring and taking steps to tackle Russia's hostile actions in its territory."
The Russian embassy however brushed off the allegations, calling them part of "incessant attempts to drive a wedge in the Russian-Bulgarian dialogue and again demonize our country."
Last year, Bulgaria expelled five Russian diplomats whom prosecutors had accused of spying, but who could not be charged because of their diplomatic immunity.
Among them was Russia’s military attaché, who had allegedly been coordinating the network in Bulgaria.
Mileva said the alleged ringleader was a former senior official in the Military Intelligence Service, who had graduated from the intelligence school in Moscow run by Russia's GRU.
Upon his return to Bulgaria, he was allegedly tasked with recruiting a network of agents with access to classified documents linked to NATO and the European Union.
The suspect's wife, who holds dual Bulgarian-Russian citizenship, allegedly acted as a contact person with the Russian Embassy where she handed over the documents, Mileva said.
Other alleged ring members include a senior Defense Ministry official involved in Ministry planning and budgeting; a military intelligence officer who compiled information on hybrid threats and risks, including from Russia; a military intelligence officer who had been sent to overseas missions; and a former military intelligence officer, who has served as military attaché abroad. Currently, he is in charge of the classified information registry of Parliament.
The six could face sentences from 10 years to life in prison if convicted, Mileva said.
"The investigation is crucial for the security of Bulgaria, but also for the security of our partners from the EU, NATO and the United States. This is the first such case since 1944," Prosecutor General Ivan Geshev said.
Bulgaria, Moscow’s closest ally during the Cold War, is a member of NATO and the EU but remains heavily dependent on Russian energy supplies.