Pius XII has long been vilified as "Hitler's pope", accused of failing publicly to condemn the genocide of Europe's Jews. Now a British author has unearthed extensive material that Vatican insiders believe will restore his reputation, revealing the part that he played in saving lives and opposing nazism. Gordon Thomas, a Protestant, was given access to previously unpublished Vatican documents and tracked down victims, priests and others who had not told their stories before.

The Pope's Jews, which will be published next month, details how Pius gave his blessing to the establishment of safe houses in the Vatican and Europe's convents and monasteries. He oversaw a secret operation with code names and fake documents for priests who risked their lives to shelter Jews, some of whom were even madeVatican subjects.

Thomas shows, for example, that priests were instructed to issue baptism certificates to hundreds of Jews hidden in Genoa, Rome and elsewhere in Italy. More than 2,000 Jews in Hungary were given fabricated Vatican documents identifying them as Catholics and a network saved German Jews by bringing them to Rome. The pope appointed a priest with extensive funds with which to provide food, clothing and medicine. More than 4,000 Jews were hidden in convents and monasteries across Italy.

During and immediately after the war, the pope was considered a Jewish saviour. Jewish leaders – such as Jerusalem's chief rabbi in 1944 – said the people of Israel would never forget what he and his delegates "are doing for our unfortunate brothers and sisters at the most tragic hour".