Under Polish law only a limited number of people, including historians and journalists, can access the files. However, the leak was sprung after Polish journalist Bronislaw Wildstein retrieved the names legally from the National Remembrance Institute (IPN) and distributed the information to colleagues. It is not known who went on to publish the list on the internet.
The IPN has said that people listed in the files are not automatically agents or collaborators and warned against using the names for political battles.
Politicians, preparing for parliamentary and presidential polls in the country, described Mr Wildstein's actions as 'irresponsible'.
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