Fashion copyists, standing by with sketch pencils and a heap of tulle poised to whip up copies of Lady Diana's wedding dress, got an unexpected preview of a dress yesterday -- possibly even THE dress.

Women's Wear Daily, trade daily of the fashion industry, published what it described as Lady Diana's wedding dress taken from a release from Lord Chamberlain's office. WWD said the release had been "slipped mysteriously under the door" of the office of Bonnie Spencer, fashion editor of a trendy trade weekly in London. The description was interpreted in a line drawing by Kenneth Paul Block on the front page of yesterday's WWD.

A Buckingham Palace spokesman said yesterday, "The story is a complete fantasy with no truth attached to it whatsoever." The spokesman also said that Sir Eric Penn, who WWD reported wrote the release, retired in February and had nothing to do with the wedding arrangements.

"We think it's authentic -- it's got the Lord Chamberlain's seal on it -- but Buckingham Palace says it's not authentic, and you have to believe what the palace says. It's a most beautiful hoax, if it is a hoax," said John Fairchild, WWD's publisher.

According to WWD, Lady Diana's dress will have a classic underdress with tailored bodice and full skirt and ruffles at the short sleeve and neck. The dress will be embellished with a marquisette overlay embroidered with a trellis of pears and crystals. According to the report, the dress will have a short train, decorated with gold embroidered flowers and a border of pale green silk lotus flowers and pink poppies.

The lace veil the bride will wear, according the WWD, was worn by Queen Victoria at her wedding to Prince Albert. But the veil, according to one wire service report, is currently on display in a London museum "and is staying here till the end of the show," a museum spokesman said yesterday.

The coronet is modern, with three bands of fluted gold supporting a garland of old gold blossoms set with diamonds and rubies, a gift to the bride from the European Economic Community. Her calf slippers will be piped in gold.

Lady Diana will also be wearing, according to WWD, a diamond-encrusted platinum bracelet, a gift from Prince Charles; an antique necklace of gold, diamond and ruby clusters, a gift from Queen Elizabeth, and saphire and diamond drop earings made by Cartier in 1923, a gift from the queen mother.

The wedding ring, according to the report, was made from a nugget of Welsh gold, and engraved in a continuous line "*Diana*Charles*29 July 1981."

London bridal firms, like Ellis Bridals, have made a sizable business in selling "knock-offs" of wedding gowns worn by royalty.According to one report, the dress Princess Margaret wore is still popular at Ellis Bridals, but Princess Anne's dress, designed by Norman Hartnell, was never a big seller.

Lady Diana's dress is expected to be an extremely popular style. Ceremonia, another London bridal gown manufacturer, has already sold more than 100 copies of the much-talked-about black taffeta strapless gown Lady Diana wore shortly after the engagement was announced.

One person staying close to the television set today is leading bridal designer Priscilla Kidder, head of Priscilla of Boston, who created the dresses for Luci Johnson and Tricia and Julie Nixon. ("Brides still want the Tricia Nixon gown today," said Kidder yesterday.) But she won't be copying THE dress "in any sense of the word," she said. "Of course, I'll be watching the wedding. I'm a traditionalist and I love weddings . . . or I wouldn't be in the wedding business."