LONDON — King Charles III and his wife, Camilla, the queen consort, will be crowned on May 6 at Westminster Abbey, Buckingham Palace announced on Tuesday, the first coronation in Britain in seven decades and one that will be pared back considerably from the extravagant ceremony held for Queen Elizabeth II in 1953. Charles is expected to shorten the length of the service, reduce the guest list and dispense with some of the more antiquated rituals, a person with knowledge of palace planning said. Those changes reflect the cost-of-living squeeze that is afflicting Britain, as well as the king’s longer-term push for a more streamlined royal family structure. The palace offered few details about the ceremony, which is still in the preliminary planning stages, but suggested that it was trying to strike a balance. “The coronation will reflect the monarch’s role today and look toward the future,” the statement said, “while being rooted in longstanding traditions and pageantry.” Charles ascended to the throne on Sept. 8 automatically on the death of his mother. He was proclaimed as king two days later in a formal ceremony conducted by an Accession Council. The coronation is a religious ritual in which the new monarch is “anointed, blessed and consecrated” by the archbishop of Canterbury.