Orlin Roussev, 46, Bizer Dzhambazov, 42, Katrin Ivanova, 32, Ivan Stoyanov, 32, and Vanya Gaberova, 29, are accused of conspiring to obtain, collect, or communicate information which might be useful "for a purpose prejudicial to the safety and interest of the state" between August 2020 and February 2023.
The three men and two women all formally entered not guilty pleas during a hearing at London's Old Bailey court.
Roussev, Ivanova and Dzhambazov also denied a second charge of possessing 18 false documents. These included Bulgarian, British, French, Croatian, Czech, Greek, Italian and Slovenian passports, a Spanish identity card and a Czech driving licence.
Tihomir Ivanchev, 38, a sixth Bulgarian who has also been accused of spying, was formally joined to the case, but did not enter a plea.
At a previous hearing, British prosecutors said the defendants were part of an organised network which had carried out surveillance and hostile action on behalf of Russia against specific targets, including for potential abductions.
Roussev's home was said to be the group's alleged operating hub in Britain, and said the network had been given tasking by a person identified as Jan Marsalek.
Marsalek, the former chief operating officer for collapsed payments company Wirecard, has not been charged with any offence in Britain but was named as a co-conspirator in the indictment. He is wanted by German police over alleged fraud and his whereabouts are currently unknown.
The case was adjourned until the next hearing on May 16 and they are due to stand trial in October.
(Reporting by Michael Holden, Editing by Louise Heavens)
