Poland's history institute says that newly seized documents suggest former president and Solidarity hero Lech Walesa was an informer.
The documents were taken earlier this week from the home of a former communist-era interior minister, Gen Czeslaw Kiszczak.
Lukasz Kaminski, head of the Institute of National Remembrance, said the documents appear authentic.
Mr Walesa has long denied being an informer in the 1970s.
The former president said the new materials could not originate from him, according to Polish radio.
The 279 pages of documents have not yet been properly analysed, and will be made public in due course, Mr Kaminski said.
Gen Kiszczak's widow had wanted to sell the documents, the institute said.
The state body prosecutes communist-era crimes.
Analysis: Adam Easton, BBC News, Warsaw
The allegation that Lech Walesa passed on information in the early 1970s to the communist secret police, before he became the hero of the Solidarity trade union, is not new.
Mr Walesa, who was cleared of the charge by a court in 2000, says the communists falsified documents about him to try to ruin his reputation.
It is unlikely these notes will change many Poles' minds about Mr Walesa because the allegation is common knowledge and he is still widely admired for his role in bringing down communism in 1989.
Mr Kaminski gave details of what he claimed was inside a file on Mr Walesa, covering the period 1970-6:
"Inside the personal file there is an envelope and inside, a hand written commitment to cooperate with the secret police signed 'Lech Walesa - Bolek'," he said.
Mr Kaminski said that among the documents in this folder were some "hand-written confirmations of receiving funds", signed with the pseudonym 'Bolek'.
"In the work folder... are many reports by a secret informant with the pseudonym 'Bolek' and notes by secret police officers from meetings with the secret informant," the director added.
Mr Walesa strenuously denied long-standing allegations of collaboration in a BBC interview in 2008.
"Nothing like that happened. I had no influence over what the secret police did and wrote," he said. "You will not find any signature of mine agreeing to collaborate anywhere," he went on.
Timeline: Lech Walesa
1943: Born in Popowo, Poland. Later trains as an electrician and starts working at the shipyards in Gdansk
1980: Becomes leader of Solidarity
1981: Arrested in anti-democracy crackdown
1982: Released, Polish martial law eased
1983: Awarded Nobel peace prize for his role in Solidarity - the first free trade union in the Soviet bloc
1988: Leads a series of nationwide strikes
1990: Elected president of Poland
1995: Defeated in presidential election
2000: Defeated again in presidential poll; cleared of security service collaboration by a special court