Pope Francis has said the time may soon come when he needs to consider stepping down—and would do so if he felt his health meant that he could not serve in the way he should, BBC reported.

He made the comments at the end of a trip to Canada—where he apologized to indigenous people—which involved extensive travel and long days.

The 85-year-old pontiff stressed that for the moment he intends to continue in his duties—and will be guided by God as to when he will step down, if he steps down at all.

"It is not a catastrophe to change Pope, it is not a taboo," he told journalists from a wheelchair on the plane from Canada's Arctic territory to Rome. "The door [to retiring] is open—it is a normal option. But until today I have not knocked on that door. I have not felt the need to think about this possibility—that is not to say that in two days' time I might not start thinking about it."

Over recent months Pope Francis has suffered ongoing knee trouble that has impacted his mobility. He spent much of his visit to Canada in a wheelchair.

But he has previously dismissed speculation about more serious, life-threatening illnesses.