Pope Francis, 85, has been forced to cancel a trip to Florence because of acute knee pain, which will also stop him presiding over Ash Wednesday celebrations, the Vatican said.

Francis suffers from sciatica, a chronic nerve condition that causes, back, hip and leg pain and has occasionally forced him to cancel official events.

The Vatican said on Friday he was victim of "an acute gonalgia, for which the doctor has prescribed a period of greater rest for the leg".

A planned trip to Florence for a meeting with Mediterranean mayors on Sunday has been scrapped, it said.

Nor will the Pope preside over the Ash Wednesday celebrations on March 2, which marks the start of the period of Lent in the Catholic church calendar.

The pontiff's health is regularly the subject of rumours within the Vatican, particularly among his critics.

Francis has dubbed his sciatica "a troublesome guest".

Last month he complained of a knee problem that prevented him from personally greeting the faithful following his weekly general audience.

He said it was "temporary", joking "I hear it happens to old people, I don't know why it happens to me".

Francis underwent surgery for an inflamed large colon last year, but is believed to be otherwise in relatively good health.