The state visit to the Vatican has been rearranged after the previous visit was postponed because of the ill health of Leo’s predecessor, Pope Francis.
The King and Queen had a private meeting with Pope Francis on their 20th wedding anniversary in April, in one of the late pope’s last meetings with high-profile visitors before he died.
The state visit on 22 and 23 October will see a series of meetings and services emphasising the warm relations between the Church of England and the Catholic church.
The King and Queen will meet the Pope and senior Vatican officials and there will be a special ecumenical service, bringing together Catholic and Anglican traditions, which will see the King and Pope praying together.
The service in the Sistine Chapel will be focused on protecting nature, in recognition of the King’s enthusiastic support of environmental causes.
But its purpose will be to show harmony between the two religious denominations, with the Sistine Chapel Choir singing alongside the Choir of St George’s Chapel and the Choir of His Majesty’s Chapel Royal.
The richly-decorated 15th Century chapel is the setting for the conclave of cardinals that elects the pope.
There have been meetings between monarchs and pontiffs before, including by the late Queen Elizabeth II.
But according to Buckingham Palace and the Church of England, there has been no similar moment, with a British monarch and pope praying alongside each other at a shared church service, since Henry VIII’s split from Rome in the 16th Century.
Royal sources have emphasised the historic significance of this occasion, with the King having a long commitment to building bridges between faiths.