Pope Francis has announced that the Vatican will open the secret archives of the era of Pius XII in March next year, which could shed light on the Catholic Church's actions during World War II.

Researchers have long sought to examine the archives over why Pius XII (1939-1958) did not intervene more against the Holocaust perpetrated by the German Nazis.

His attitude has often been denounced by critics, including Jewish groups, as a form of passive complicity.

"I decided that the opening of the Vatican Archives for the pontificate of Pius XII would take place on March 2, 2020," the pontiff said.

The date is the 81st anniversary of the election of Eugenio Pacelli to the papacy.

"The Church is not afraid of history," added the pope, recalling that Pius XII found himself as head of the Roman Catholic Church "at one of the saddest and darkest times of the 20th century". 

Pope Francis said he made the decision understanding that serious historical research will evaluate "in a fair light, with appropriate criticism the moments of exaltation of this pope and, no doubt also moments of serious difficulties, tormenting decisions, and Christian and humane care".

For many historians, Pope Pius XII could have condemned more forcefully the massacre of Jews by the Nazis, but he did not do it out of diplomatic caution and in order not to put Catholics in danger in occupied Europe.

Pope Francis also said that the wartime pope's decisions may appear to some as "reticence" and were instead attempts "to maintain, in times of the deepest darkness and cruelty, the small flame of humanitarian initiatives of a hidden but active diplomacy".

The Vatican was officially neutral during the war.

Other historians point out that Pius XII saved tens of thousands of Italian Jews by ordering convents to open their doors to take them in.

The perception of passivity was in part fostered by a play, "The Vicar", by German playwright Rolf Hochhuth in 1963, which was later adapted by Greek director Costa-Gavras in the 2002 film "Amen", which conveyed the image of Pius XII as a weak pope.

While other popes such as John Paul II have been canonised as saints, the first step in the process, beatification, for Pius XII was relaunched in 2009 by Pope Benedict XVI, but it has completely stalled due to the controversies surrounding his wartime papacy.