The 85-year-old announced the decision in Latin during a meeting of Vatican cardinals this morning.
He said that because of his age and diminishing strength, he didn't feel he could carry on.
On the Island, Lord Bishop Robert Paterson has described the Pope as having a formidable intellect but also formidably conservative.
He admits the resignation has come as a surprise:
500% increase in immigration arrests on the Isle of Man
Due to targeted activity rather than increased criminality
More taxpayers' cash could be spent on 'cleaner heating'
DEFA seeking Tynwald approval for new scheme
RAF veteran says assisting blind climber will be 'massive honour'
Gary helping guide 87-year-old up Ben Nevis
Financial leaders from small countries visiting Island
Delegates from 25 small nations attending seven-day event
