The pope has said he would be ready to resign one day if serious health problems prevented him from running the Catholic Church.

This slowing down period of Pope Francis’s papacy has been brought on by his increasing age and difficulty walking.

The 85-year-old told reporters on the plane back to Rome following a week-long holiday in Canada that he doubts he can keep up the same pace of travel.

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In Canada, the pontiff had come to apologize for the Catholic Church’s role in the mistreatment of indigenous children in Catholic schools.

Because of a fractured and inflamed ligament in his knee, he has had to use a wheelchair, cane, or walker for the past three months.

“I think that at my age and with this limitation I have to preserve myself a bit in order to be able to serve the Church, or decide to step aside,” Francis said.

He has previously said he could follow in the footsteps of Pope Benedict, who in 2013 became the first pope in 600 years to resign instead of rule for life.

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“It’s not strange. It’s not a catastrophe. You can change the pope,” he said.

“The door is open. It is one of the normal options. Up until today, I did not use that door. I did not think it was necessary to think of this possibility but that does not mean that the day after tomorrow I don’t start thinking about it.”

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He added: “This trip was a bit of a test. It is true that I can’t make trips in this condition. Maybe the style has to change, make fewer trips, make the trips I have promised to make, re-jig things. But it will be the Lord who decides. The door is open.”

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During his trip to Canada, the pope donned an indigenous feathered headdress before going on to say the forced assimilation of native peoples into Christian society destroyed their cultures and severed their families.

He apologised for Christian support of the “colonising mentality” of the times” and added: “With shame and unambiguously, I humbly beg forgiveness for the evil committed by so many Christians against the indigenous peoples.”

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More than 150,000 native children in Canada were forced to attend state-funded Christian schools from the 19th century until the 1970s in an effort to isolate them from the influence of their homes and cultures in an effort to “Christianise” and assimilate them into mainstream society.