The EU president says Polish intelligence probe is a part of "smear campaign".
EU president Donald Tusk on Wednesday testified for eight hours in Warsaw as a witness in an investigation of two heads of military counter-intelligence (SKW) suspected of cooperating with foreign intelligence services without his official permission.
A former SKW head has said that Tusk, who was Poland's prime minister at the time and thus ultimately responsible for the intelligence service, was fully aware of secret services' cooperation with Russia and had authorised it.
Speaking to reporters after the questioning, the former liberal Polish prime minister said: "This whole affair is extremely politically motivated."
The 59-year old EU official is the ex-leader of Poland's Civic Platform (PO), now the largest opposition party, and an arch-rival of Jaroslaw Kaczynski, head of the ruling eurosceptic Law and Justice (PiS) party.
The prosecutors who summoned him to testify are under the direct control of the PiS-run government.
He had been summoned for closed-door questioning in March but was unable to appear because of a previous European Parliament commitment.
The original summons came days after EU leaders gave Tusk another term as president despite strong opposition from Poland, which cast the sole vote against him. An angry Warsaw accused the EU of "cheating".
Tusk warned that he would use his EU immunity if he came to the conclusion that the investigation was "deliberately used to make it more difficult or impossible for me to exercise my mandate as EU president."
The probe centres on Generals Janusz Nosek and Piotr Pytel, who are suspected of having "overstepped their duties by collaborating with services of a foreign state" without authorisation.
According to Polish media reports, at issue is an agreement struck with Russia's FSB intelligence service — the successor to the KGB — shortly after a 2010 plane crash in Russia which killed Polish president Lech Kaczynski and scores of senior Polish officials.
Some reports suggest the agreement was aimed at cooperating on the withdrawal of Polish troops from NATO operations in Afghanistan at the time, while others say it allowed FSB agents to visit the headquarters of Poland's SKW military counter-intelligence agency unfettered.
Kaczynski has accused Tusk of "moral responsibility" for the death of his brother Lech in the plane crash.