"Documents related to the collaboration of a secret agent codenamed 'Bolek' were shown to Lech Walesa," said the Institute of National Remembrance (IPN), which prosecutes Nazi and communist-era crime.
The 72-year-old "denied the authenticity of the documents," the IPN said in a statement, adding that it would now go ahead with handwriting analysis to see if the Nobel Peace laureate's signature matches that of the files.
Walesa has been battling the allegations since February, when the IPN seized the previously unknown regime documents from 1970-76 from the widow of a communist-era general.
He enigmatically admitted however to having "made a mistake" and in the past had said he signed "a paper" for the secret police during one of his many interrogations.
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Experts have consistently raised doubts about the credibility of communist secret police files, arguing they could easily have been manufactured to frame opposition activists like Walesa.
A special vetting court ruled in 2000 that there was no basis to suspicions that he had been a paid regime agent.
A book published by the IPN in 2008 alleged that while the regime registered Walesa as a secret agent in December 1970, he was cut loose in June 1976 due to his "unwillingness to cooperate".
Poles have mixed feelings about Walesa. His boldness in standing up to the communist regime is still widely respected, but the combative and divisive tone of his later presidency earned him scorn in many quarters.