The Roman Catholic Church in Poland has said that about one dozen of the country's 132 bishops were registered as informers during the communist era.
Bishop Stanislaw Wielgus' past was first revealed in newspapers
A Church commission found that secret police files from the period classed most of them as secret collaborators.
One of the bishops was listed as an intelligence service agent.
The Church requested the enquiry after Warsaw's new Archbishop Stanislaw Wielgus resigned, confessing he had collaborated with the secret police.
Since then, other prominent Church figures have also stepped down from their positions.
None of the 12 bishops whose files revealed their contact with the secret police have been named.
A top bishop said that that the files were so confused that it was not clear exactly what role each of those identified as collaborators had played.
Correspondents say the revelations of collaboration have severely weakened the Church, to which 90% of Poland's 40 million people belong.
The Church was highly esteemed because of its leading role in the fight against communism in Poland and worldwide, particularly during the time of Polish Pope John Paul II.
But historians estimate that up to 15% of Polish clergy agreed to inform on their colleagues in the communist era.