A Polish institute responsible for prosecuting communist-era crimes said Thursday that new-found documents showed Lech Walesa was a communist secret agent, an allegation the freedom hero immediately denied.

In a secret police file "there is a collaboration agreement signed by Lech Walesa (codename) 'Bolek'," Lukasz Kaminski, head of the Institute of National Remembrance (IPN), told reporters in Warsaw.

Walesa denied the allegation in a Thursday social media post saying: "There cannot be any materials written by me."