In a break with tradition, Lady Diana Spencer will not promise "to obey" Prince Charles when they marry at St. Paul's Cathedral on July 29, Buckingham Palace announced today.

But like most other English brides, she will pledge to "love him, comfort him, honor and keep him, in sickness and in health. . . "

Leaving out "obey" was the couple's own decision, said Archbishop of Canterbury Robert Runcie, who will perform the ceremony.

The 70-minute service, which Prince Charles and Lady Diana Spencer have helped plan, will both preserve and break other royal and religious traditions while displaying the talents fo British composers and musicians.

After music by Edward Elgar, Henry Purcell, Benjamin Britten and Ralph Vaughan Williams, among others, and trumpeted marches and fanfares greeting the arrivals of the groom and bride, the service will begin at 11 a.m. under the dome of St. Paul's with the familiar words: "Dearly beloved, we are gathered here in the sight of God and in the face of this congregation, to join together this man and this woman in holy matrimony." t

The words come from the three-century-old Book of Common Prayer of the Church of England, rather than the new Alternative Service Book, which has caused considerable controversy in the Anglican church with its emasculation of such flowery language in the cause of modernity.

In the traditional wedding service chosen by Charles and Diana, the Very Rev. Alan Webster, dean of St. Paul's Cathedral, will even declare to the congregation of royalty, heads of state and nobility: "Therefore if any man can shew any just cause, why they may not lawfully be joined together, let him now speak, or else hereafter for ever hold his peace."

According to the order of service made public by Buckingham Palace yesterday on Diana's 20th birthday, the archbishop of Canterbury will then ask the groom, addressed as Charles Philip Arthur George, and the bride, as Diana Frances, if each will love, comfort, honor and keep the other, in sickness and health, forsaking all other, "so long as ye both shall live?"

Also following Anglican tradition, the groom and bride will declare these vows themselves and marry each other, with Charles concluding by saying, "With this ring I thee wed: with my body I thee honor: and all my worldly goods with thee I share: in the name of the Father, and of the Son, and of the Holy Ghost. Amen."

Another break with tradition will be made with modern-language prayers from the Alternative Service Book red by the present and former archbishops of Canterbury and also by the Catholic archbishop of Westminster, the moderator of the church of Scotland and a church of Wales clergymen -- a show of ecumenicalism unprecedented for royal weddings here.

A Welsh composer and the organist of St. Paul's have composed specially commissioned music for the latter part of the service, which ends with Elgar's famous "Pomp and Circumstance" and William Walton's majestic "Crown Imperial" march, followed by a final fanfare of trumpets.

The music will be performed by members of the Royal Opera House Orchestra, the English Chamber Orchestra and the Philharmonia Orchestra of London, the Choir of St. Paul's Cathedral, the Bach Choir of London, the Choir of Her Majesty's Chapels Royal, New Zealand Opera soprano Kiri Te Kanawa, and the trumpeters of both the Queen's Household Cavalry and the Royal Military School of Music.

Prince Charles personally chose most of the music and the performers. An avid music lover who plays piano and cello, he is a patron of all three orchestras chosen. He also is president of an occasional singer with the 200-voice Bach Choir, in which his royal cousin, the Duchess of Kent, will be singing at the wedding.

The Welsh influence in parts of the ceremony also reflects the interest and sense of duty about Wales that Charles has shown since studying Welsh language and culture before being invested as prince of Wales, the most important of his many titles as heir to the British throne. Lady Diana will immediately become princess of Wales when Charles puts the wedding ring on her finger.