The Spanish government was divided after a report was published alleging that dozens of Catalan separatists had been subject to surveillance through spyware.
The government has "nothing to hide" and wants to cooperate with the judiciary, spokesperson Isabel Rodríguez said on Tuesday.
Defence Minister Margarita Robles is due to answer questions in parliament, she added, stating that she could not provide further details about what she called a "matter of national security."
However, she confirmed that no conversations were intercepted in Spain without a court order.
Research by Canadian research group Citizen Lab published by the New Yorker magazine claimed that more than 60 Catalan separatist leaders had been subject to systematic monitoring, as had some of their friends and families.
Mobile phones belonging to politicians, lawyers and activists were hacked and bugged between 2017 and 2020 with Israeli spyware Pegasus.
Those subject to the surveillance included Catalan regional president Pere Aragonès and his three predecessors, Quim Torra, Carles Puigdemont and Artur Mas.
Puigdemont said he would take legal action against all those responsible, in Spain, Germany, France, Luxembourg and Switzerland.
He called on European Commission President Ursula von der Leyen to "act urgently" and hold Spain accountable for using Pegasus against political opponents, slamming this as a "massive violation of fundamental rights."
Among others, the junior partner in Prime Minister Pedro Sánchez's governing coalition, Unidas Podemos (UP), also called for "clear explanations" and a thorough investigation. If necessary, "heads must roll", UP spokesperson Pablo Echenique said.
In the article "How Democracies Spy on Their Citizens," the New Yorker states that Pegasus spy software from Israeli company NSO Group was used in the surveillance.
NSO Group claims that the programme is only sold to law enforcement agencies and intelligence services.
The Citizen Lab at the University of Toronto investigated Whatsapp vulnerabilities in connection with espionage.