Jaroslaw Kaczynski missed out part of the oath as he appeared before the commission, which is looking into whether the previous government illegally hacked the phones of targets including political opponents, amid a growing scandal over the use of Pegasus spyware.

The commission said it would apply to a court for Kaczynski to be fined for refusing to take the oath in full.

As party leader Kaczynski was seen as Poland's de-facto ruler during much of the Law and Justice (PiS) party's eight years in power which ended in 2023, although he did not hold any official government position for much of the time.

The appearance of the once seemingly untouchable eminence grise of Polish politics before the commission was the most potent symbol yet of the new pro-European government's determination to hold those it accuses of wrongdoing to account.

Kaczynski justified his refusal to swear a complete oath by referring to an article of the rules on parliamentary commissions which says that a witness cannot reveal secret or top secret information without getting permission from the relevant authority.

"I do not promise to say everything I know, because I simply cannot do so without permission from the prime minister," Kaczynski said.

He said that to his knowledge Pegasus had not been used against high-ranking state officials and that he had not attached much importance to it.

Reports in 2021 by the Associated Press that the software, developed by Israel-based NSO Group, was used to hack the phones of government critics, including the head of the election campaign of what was then Poland's largest opposition party, drew accusations that security services eroded democratic norms.

However, more recently the accusations have taken on a new dimension due to media reports that members of the former ruling party were themselves victims of the phone hacking.

If confirmed, such reports could blow apart an opposition that has thus far been united in the defence of its record and the actions of ministers that the new government says broke the law.

(Reporting by Alan Charlish; Editing by Susan Fenton)