By Philip Pullella

ROME, (Reuters) - Pope Francis revealed in a new interview published on Sunday that after he was elected in 2013 he signed a letter of resignation to be used if someday severe and permanent health problems made it impossible to carry out his duties.

Francis, who turned 86 on Saturday and appears to be in good health except for a knee ailment, made the comment in an interview with the Spanish newspaper ABC.

Francis said he gave the letter to then Vatican Secretary of State Cardinal Tarcisio Bertone, who was a holdover from the previous papacy of Benedict XVI. Bertone remained in the position for about six months after Francis was elected on March 13, 2013.

Francis has often said that he would resign if health impaired him from running the 1.3 billion-member Roman Catholic Church.