Christian leaders are praying for the Queen after Buckingham Palace issued a statement today saying that her doctors are "concerned" for her health.
The Queen is at her summer residence, Balmoral, in Scotland, where her doctors have recommended that she "remain under medical supervision".
"The Queen remains comfortable and at Balmoral," the statement added.
The Archbishop of Canterbury said he was praying God's comfort for the Queen, her family and all those caring for her.
"My prayers, and the prayers of people across the Church of England and the nation, are with Her Majesty The Queen today," he said.
"May God's presence strengthen and comfort Her Majesty, her family, and those who are caring for her at Balmoral."
Members of the royal family, including heirs to the throne Prince Charles and Prince William, are travelling to Balmoral to be with her.
The BBC reports that Prince Harry and Meghan, the Duke and Duchess of Sussex, are also on their way to Scotland. They have been in Europe for the Invictus Games and a number of other public engagements.
Cardinal Vincent Nichols, head of the Catholic Church in England and Wales, said he was "concerned" to hear the news of the Queen's health.
"May the blessings of the Lord strengthen and comfort her and her family," he said.
The Free Churches Group has issued the following prayer: "Dear God, our Father, we are thankful this day for the blessings and mercies that You bestow on us. We remember at this time Her Majesty The Queen and her family. We pray for the health of The Queen. May You bless and keep her. Grant to her family and the nation your strength and peace today. These things we ask in the name of Jesus. Amen."
The office of the Anglican Communion said, "We join with Anglicans around the world in praying for Her Majesty Queen Elizabeth II, and her family.
"The Queen is Head of State of 17 countries; and has been Head of State of others during her life. She is also Head of the Commonwealth. All of these areas are served by one or more of the 42 Churches of the Anglican Communion.
"We pray for God's presence to strengthen and support The Queen, the Royal Family, and those who are caring for Her Majesty."
The Rt Rev Dr Iain Greenshields, Moderator of the General Assembly of the Church of Scotland, "All of us in the Church of Scotland are deeply concerned to hear reports about Her Majesty the Queen's health.
"Our thoughts and prayers are with her and the entire Royal Family at this worrying time."