"I informed parliament's legal affairs committee that I have started a preliminary investigation over tapping of a mobile phone of the chancellor," Prosecutor Harald Range said after addressing the committee.

The NSA is not going to figure in the probe for now, as the case on espionage is opened against the "unknown" suspected party.

Prosecutors will also continue to monitor the “possible massive collection of telecommunications data of German citizens by British and American intelligence services.” Efforts against “cyber-espionage” will be intensified.

Earlier, German media said the investigation was likely not to take place at all. Range was reported to believe the proceedings would be purely “symbolic” since it is impossible to gather evidence about the activities of the NSA and its British counterpart, GCHQ, on German soil.

The probe has indeed been a long-anticipated one, as Snowden’s leaks published in various European and US media revealed that Germany has been the NSA’s most-spied-on EU country as early as in June 2013. Germany’s BND intelligence service was also said to send “massive amounts” of phone and email data to the NSA in August. In October it was alleged that some 35 world leaders, including Merkel, were bugged by the NSA.

Merkel, who initially justified the spying practices of Germany’s ally, was enraged when she learned she was allegedly on the NSA tapping list since 2002.

Edward Snowden, the US national behind the NSA leaks, said that he is ready to testify before the Bundestag over the alleged US surveillance if he is enabled to provide evidence to a parliamentary committee – which the German government has so far been unwilling to accept.

However, the whistleblower also criticized what he called a “Merkel effect” when referring to the similar situation in the US Congress, in which Senator Dianne Feinstein lashed out at the Central Intelligence Agency for allegedly spying on Congressional committee members, despite her earlier apologetic stance towards the NSA.

German Chancellor Angela Merkel (AFP/DPA)

“It’s equally if not more concerning that we’re seeing another ‘Merkel Effect,’ where an elected official does not care at all that the rights of millions of ordinary citizens are violated by our spies, but suddenly it’s a scandal when a politician finds out the same thing happens to them,” Snowden told NBC News.

Despite seemingly cooling down relations between Berlin and Washington, the NSA leaks have until now sparked only a number of statements from top German officials, who called the alleged spying "unacceptable."

“Extensive preliminary investigations have established that sufficient factual evidence exists that unknown members of US intelligence services spied on the mobile phone of Chancellor Angela Merkel,” Range's office said.