Prince Charles and Camilla Parker Bowles will apologise for their sins and wickedness and ask for forgiveness at their marriage ceremony, 34 years and two failed marriages after they first fell in love.
Supporters of Charles' first wife, the late Diana, Princess of Wales, will say their repentance comes too late.
But it is a significant move by the couple who chose a forceful passage of penitence from the traditional 1662 Book of Common Prayer, rather than a softer modern reading, to seal their union before God.
After their civil ceremony in Windsor Guildhall, the newlyweds will have their marriage blessed at Windsor Castle by the Archbishop of Canterbury in a service in which they and the congregation will read an acknowledgement of their sins.
Charles, 56, and Camilla, 57, will also promise to be faithful to each other, as they both did to their first spouses.
But their adulterous affair killed off Charles' marriage to Diana, who famously said "there were three of us in this marriage".
Parker Bowles' 22-year marriage to Andrew Parker Bowles ended in 1995, a year before the Prince and Princess of Wales divorced.
The royal couple's bid for forgiveness could be aimed at conservative members of the Church of England who questioned the validity of a civil wedding between two divorcees.
The simple half-hour ceremony conducted by registrar Clair Williams in front of 30 guests will include no mention of God, nor have prayers or hymns.
It will not be attended by the Queen.
Once they leave the Ascot Room in the Guildhall as Their Royal Highnesses the Duke and Duchess of Cornwall, Charles and Camilla and their 30 closest family and friends have 90 minutes to kill before the service of prayer and dedication at St George's Chapel, Windsor Castle, starts at 2.30pm.
The passage of penitence
Almighty God, father of our Lord Jesus Christ, Maker of all things, Judge of all men: We acknowledge and bewail our manifold sins and wickedness, Which we, from time to time, most grievously have committed, by thought, word and deed, Against thy Divine Majesty, Provoking most justly thy wrath and indignation against us. We do earnestly repent, And are heartily sorry for these our misdoings; the remembrance of them is grievous unto us; the burden of them is intolerable. Have mercy upon us, have mercy upon us, most merciful Father; For thy Son our Lord Jesus Christ's sake, Forgive us all that is past; And grant that we may ever hereafter Serve and please thee In newness of life, To the honour and glory of thy name; Through Jesus Christ our Lord.