The Roman Catholic Church in Poland says the new archbishop of Warsaw was a collaborator with the former communist regime, amid an escalating row.
The scope of Father Wielgus's alleged collaboration is unclear
Stanislaw Wielgus has been at the centre of a furore in Poland since a newspaper questioned his past after his appointment by the Vatican.
The archbishop took office earlier and will be confirmed at Sunday Mass.
He has admitted having had contacts with security agents but denied informing on priests.
He repeated on Friday: "I did not carry out any intelligence task. I never inflicted any harm on anyone." He said documents suggesting otherwise were drawn up only by communist "functionaries".
Polish ombudsman Janusz Kochanowski said the files he examined showed that Archbishop Wielgus collaborated with the Sluzba Bezpieczenstwa (SB) secret police in the 1970s.
Then on Friday the Church's own investigative committee said: "There exist numerous, important documents which show that Father Stanislaw Wielgus said he was ready to collaborate, in a conscious and secret manner, with the communist security services, and that he had begun that collaboration."
It went on to say that there was no clear proof that he caused anyone any harm.
The revelations have triggered calls for his resignation.
The BBC's Adam Easton in Warsaw says Poland's communist authorities were particularly anxious to infiltrate the Catholic Church because they viewed it as a centre of resistance, but most priests refused to inform on their colleagues.
Last month Pope Benedict XVI appointed Stanislaw Wielgus Archbishop of Warsaw, one of the most senior positions in the Polish Catholic Church.
Just before Christmas, the Vatican released a statement insisting the Pope had been fully briefed on Archbishop Wielgus' past and supported his appointment.
The Church has a very prominent role in Polish society and 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.
Father Wielgus says he - like all priests - was obliged to meet SB agents.
But Polish media have alleged that he was more involved than most, and was given "special training for agents".
Historians estimate up to 15% of Polish clergy agreed to inform on their colleagues in the communist era.