Letters to Princess Anne from a handsome aide have raised speculation of a royal divorce - an occurrence that has wrought constitutional upheavals and brought heartbreak to Queen Elizabeth II's family.

Whether Anne's 15-year marriage to Capt. Mark Phillips is really on the rocks or is simply one of those long-distance unions not unusual in the British upper classes remains open to question.The queen's only daughter and Phillips, a commoner and former army captain who runs equestrian centers, lead virtually separate lives. They were apart for more than half the time last year. They have two children - Peter, 11, and Zara, 7.

News of the mysterious collection of letters to Anne, 38, from Royal Navy Cmdr. Timothy Laurence, a 34-year-old bachelor and equerry at Buckingham Palace, gave new life to rumors which began in 1981 that her marriage is in trouble.

The four stolen letters were delivered last week to a mass circulation newspaper, The Sun, which did not publish them. Police are investigating the theft.

With panting headlines, the tabloid press said Anne and Laurence are in love.

As usual, the principal characters said little or nothing.

Anne carried on with a punishing schedule of routine royal engagements. Phillips, 40, went off to Spain to see about setting up a riding center there, and Laurence dropped out of sight.

Anne is only seventh in line to the throne - behind her brothers Charles, Andrew and Edward, two young nephews and a baby niece - so her divorce would hardly bring a constitutional crisis.

It was not always so.

Divorce has hung over the throne since the six-times married Henry VIII broke with the Roman Catholic Church in 1534 because the Vatican would not annul his marriage to Catherine of Aragon.