Recently discovered documents appear to show that former Polish President Lech Walesa signed receipts showing he was a paid informant. Walesa has denied the accusations.
According to the head of the National Remembrance Institute (IPN), Lukasz Kaminski, documents seized this week include a commitment to provide information from 1970-76, signed by Walesa with the codename "Bolek." There are also pages of reports and receipts for money signed by Walesa during this period.
The IPN is responsible for prosecuting Communist-era crimes.
A hugely significant figure in Poland's transition from Soviet satellite state to European republic, Walesa has denied the accusations. He admitted he signed a commitment to be an informant during the 1970s but never acted on it. He was cleared by a special court 16 years ago. The court ruled in 2000 that there was no evidence of his collaboration with secret services.
Soviet authorities commonly used false information to discredit activists while secret services used personal information to blackmail targets and keep dissidents under control.
Speaking from Venezuela, where he is traveling, the 72-year-old Walesa suggested on Thursday that the papers were fake.
"There can exist no documents coming from me. I will prove that in court," he said.
The 279 pages of documents were seized this week from the house of the late General Czeslaw Kiszczak, the last Communist-era interior minister. Kiszczak helped orchestrate a crackdown on the Solidarity movement in 1981. He died last year.
On Tuesday, Kiszczak's widow offered to sell documents concerning secret informer "Bolek" to the IPN. She demanded 90,000 zlotys ($23,000, 20,000 euros). Prosecutors seized the documents the same day under a law requiring important historic papers be handed in to the authorities.
Five more packets
Kaminski said the documents appeared to be authentic and would be made public in due course. Five more packets of seized documents have not yet been opened, he said, adding that historians needed time to analyze the contents.
Walesa founded and led Solidarity, a workers-inspired movement, from 1980. He later served as president of Poland from 1990 to 1995. Last year, Walesa launched a scathing attack on the government, calling for early elections.
jm/sms (dpa, AFP)