London-based Katrin Ivanova, 33, Vanya Gaberova, 30, and Tihomir Ivanchev, 39, were convicted of conspiracy to spy at London’s Old Bailey court at the end of a trial which lasted more than three months.
They spied on individuals and locations “that were of interest to the Russian state”, according to the crown prosecution service (CPS), which brings prosecutions in Britain.
The espionage at the behest of Moscow took place over three years between August 2020 and February 2023, the court heard.
Ivanova was also found guilty on charges of possessing false identity documents with improper intention.
They are due to be sentenced in May, and could face jail terms of up to 14 years.
Metropolitan police counter-terrorism chief commander Dominic Murphy told PA news agency this was a case of “industrial-scale espionage on behalf of Russia”.
“This is one of the largest and most complex examples of a group working for a foreign state to conduct intelligence surveillance operations here in the UK,” Murphy added.
They were part of a spy ring of six Bulgarians, including ringleader Orlin Roussev, 47.
Roussev and Bizer Dzhambazov, 43, had previously pleaded guilty to spying, as well as a sixth defendant Ivan Stoyanov, 32.
The six-person cell targeted journalists and a Kazakh former politician, and plotted to kidnap and honeytrap targets, following them across several European cities.
The group planned “disruptive activity” at the Kazakh embassy in 2022, discussing a plan to spray the building with fake pig’s blood.
It also carried out surveillance on a US military base in Germany the same year.
“They compiled detailed reports on their targets and were paid significant sums of money for their work,” according to the CPS.
Ringleader Roussev received his instructions from Jan Marsalek, an Austrian fugitive who reportedly fled to Russia in 2020 after becoming a wanted man in Germany over fraud allegations.
Marsalek, the former chief operating office of payments firm Wirecard, was acting as a proxy for Russian intelligence services, according to the CPS.
One spy operation targeted investigative journalist Christo Grozev, from the Bellingcat website, who uncovered Russian links to the 2018 Novichok chemical weapon attack in the English town of Salisbury and the drowning of a Malaysia Airlines plane four years earlier.
Another focused on Roman Dobrokhotov, a UK-based Russian investigative journalist and dissident who founded the news website The Insider.
“This was a high-level espionage operation with significant financial rewards for those involved in the spy ring,” Frank Ferguson from the CPS said.
“This prolonged activity also undermined the security and safety of the UK; and there can be no doubt that each of the defendants knew exactly who they were spying for,” Ferguson added.