Pope Pius XII told senior bishops during World War II that, should he be arrested, his resignation would become effective immediately and the bishops should flee to a friendly country to re-establish church leadership and appoint a new pope.
The bishops would be expected to flee to a safe country, probably neutral Portugal, documents in the Vatican's Secret Archives show, the UK Telegraph reports.
"Pius said 'if they want to arrest me they will have to drag me from the Vatican'," said Peter Gumpel, the German Jesuit priest who is in charge of researching whether Pius should be made a saint, and therefore has access to secret Vatican archives.
Pius XII told his advisers "the person who would leave under these conditions would not be Pius XII but Eugenio Pacelli" - his name before he was elected Pontiff - thus giving permission for a new Pope to be elected.
"Pius wouldn't leave voluntarily. He had been invited repeatedly to go to Portugal or Spain or the United States but he felt he could not leave his diocese under these severe and tragic circumstances," Fr Gumpel said.
Vatican documents, which still remain secret, are believed to show that Pius was aware of a plan formulated by Hitler in July 1943 to occupy the Vatican and arrest him and his senior cardinals.
On 6 September 1943, days after Italy signed the September 3 armistice with the Allies and German troops in occupied Rome, Pius told key aides that he believed his arrest was imminent.
General Karl Otto Wolff, an SS general, was told to "occupy as soon as possible the Vatican, secure the archives and art treasures and transfer the Pope, together with the Curia so that they cannot fall into the hands of the Allies and exert a political influence."
Some historians have claimed that General Wolff tipped off the Vatican about the kidnap plans and that he also managed to talk the Fuhrer out of the plot because he believed it would alienate Catholics worldwide.
Vatican planned to move to Portugal if Nazis captured wartime Pope (UK Telegraph)