Secret surveillance has long been a hot topic in the Balkan country, which joined the European Union in 2007.
The European Court of Human Rights (ECHR) has said that Bulgaria violates the European Convention of Human Rights when it comes to secret surveillance and retention and accessing of communication data.
An ECHR chamber ruled on Tuesday that Bulgarian laws about retention and accessing communication data were incapable of limiting such operations to what was strictly needed.
"The European Court of Human Rights held, unanimously, that there had been a violation of Article 8 (right to respect for private life and correspondence) of the European Convention on Human Rights," it said in a statement.
The court found that there was a lack of proper judicial oversight over decisions to issue warrants for surveillance in Bulgaria.
A lack of clear regulation had led to a situation where secret surveillance data could be used for nefarious purposes, it said.
The case was brought by two Bulgarian lawyers and two rights protection non-profit organisations in 2012.
A special parliamentary commission found in August that special services eavesdropped on over 900 Bulgarians, including politicians, journalists and activists between the start of anti-graft protests in 2020 until May 2021.
The head of the commission then, Nikolai Hadzhigenov, has said classified data pointed to the biggest wire-tapping and eavesdropping on Bulgarian citizens in the recent history.
The prosecutors' office has denied any illegal eavesdropping on politicians.