Former Archbishop of Canterbury Lord Carey has told the Times newspaper he thinks the Prince of Wales and Camilla Parker Bowles should marry.
New rules mean the prince could marry Mrs Parker Bowles in church
He said he thought it would be "the natural thing" for them to do.
Lord Carey also met Mrs Parker Bowles "several times" after details of her relationship with the prince became known, the paper reports.
The Church of England recently revised its rules to allow divorcees to remarry in church in some cases.
Until the change, Prince Charles could not have married Mrs Parker Bowles in church, as her former husband Andrew is still alive.
Church Society chairman George Curry told BBC News Charles' marriage to Mrs Parker Bowles would be an "adulterous union... according to the teachings of Jesus Christ".
However, Lord Carey told the Times the prince was more "sinned against than sinning" in his marriage to Diana, Princess of Wales, and should now be able to remarry.
"The Christian faith is all about forgiveness," the former archbishop told the newspaper. "We all make mistakes.
"There is no doubt that there has been a strong loving relationship, probably since they were very young.
"He is the heir to the throne and he loves her. The natural thing is that they should get married."
Lord Carey and his wife Eileen met Mrs Parker Bowles at their son Andrew's south London house, after becoming concerned that she was being demonised by the media, the Times says.
The former archbishop calls Mrs Parker Bowles "very nice... very bright, able, astute, tough".
"I got very concerned that she was an ogre in the eyes of the media," he adds.
And Diana was "a little more cunning at using the media".
But royal author, Christopher Wilson, told BBC News that although Lord Carey would have loved to have been able to marry Charles to Mrs Parker Bowles, the Church of England was still "not keen on the idea of
people who have been responsible for the breakdown of a marriage" remarrying in church.
And as well as Charles' marriage to Diana, Mrs Parker Bowles' own marriage had also broken down, "which was also the responsibility... of the Prince of Wales".
Charles had a deep desire to marry Mrs Parker Bowles, Mr Wilson added.
But he would always be "tarred by the brush of" Diana, and was too concerned about public opinion.