Greek President Katerina Sakellaropoulou called on Tuesday for an investigation into the tapping of a political leader's phone by the intelligence service (EYP).
The scandal broke last week amid growing concern in the EU about the use of spyware software and sparked uproar at home, with opposition parties labelling the revelations Prime Minister Kyriakos Mitsotakis's personal Watergate.
In a statement, Sakellaropoulou said that protecting the right to privacy was "a fundamental condition of a democratic and liberal society" and that the respect of democracy transcends politics.
"It requires the immediate and full clarification of the wiretapping case," she said.
The leader of Greece's Socialist PASOK party and member of European Parliament, Nikos Androulakis, said on Friday he had learned that EYP was listening to his conversations last year.
Earlier that day, EYP's chief and Prime Minister Kyriakos Mitsotakis's chief of staff were sacked.
A government spokesman said that EYP had tapped Androulakis's phone but that the surveillance, which was approved by a prosecutor, was lawful and the prime minister was informed about it last week.
The government has not said why Androulakis's phone was hacked.
In a public address on Monday, Mitsotakis said that if he had known he "would have never allowed it".
PASOK is Greece's third-largest political party and was for decades the main political rival of Mitsotakis's conservative party, New Democracy.
The government has said it will back a request by the opposition for a parliamentary investigating committee on the issue.
The European Commission is also monitoring the case. Cypriot MEP George Georgiou, vice-chair of the EU's PEGA committee investigating malware surveillance software, has also sent a letter to the committee proposing a mission to Greece to investigate the allegations.
(Reporting by Renee Maltezou and George Georgiopoulos in Athens, Michele Kambas in Nicosia and Charlotte Van Campenhout in Brussels; Editing by Nick Macfie)
