Pope Francis announced on Tuesday he would declare the use and possession of nuclear weapons "immoral" in the Catholic Church's official manual, after visiting the Japanese cities of Hiroshima and Nagasaki where atomic bombs were dropped during World War II.
The cities are the only areas to have been targeted by nuclear weapons, the 1945 attacks by the United States killing 224,000 people.
The catechism is the church's official instruction book, which summarises the principles of the faith.
"I have said that the use of nuclear weapons is immoral," the pontiff told reporters during his flight back to Rome.
"This must go into the catechism of the Catholic Church, not just use but also possession," he said, adding that "a government's madness can destroy humanity".