The relentless public scrutiny in Britain of Diana, princess of Wales, may have reached a turning point today when one of the country's racier tabloids, the Daily Star, admitted that most of the horror stories about her are nonsense and declared, "Enough Is Enough."

After weeks in which reports in the Star and other mass-circulation dailies tended to portray Diana as a neurotic brat trapped in an increasingly unhappy marriage, the newspaper said, "Have we all lost our sense of balance? We have certainly lost our sense of fair play." The royals will always be news, the Star said, but "fiction" and "rubbish" go too far.

The limit could well have been reached Sunday when The Mail featured an article purporting to quote an American psychiatrist saying Diana was "heading for a breakdown." The News of the World carried a "psychiatrist's casebook," which concluded that Diana is "unhappy and lonely."

"I knew this would happen," said Dr. Thomas Holms, one of the psychiatrists quoted in the London papers. Holms, reached today in Seattle, denied diagnosing the princess' mental condition.

Holms, a professor at the University of Washington Medical School,said he tabulated Diana's score on the Holms-Rahe Life Change Inventory, a well-used American device for measuring stress.

"We just took common facts about her life that were public knowledge--she did get married, she was pregnant, there was an addition to her family, she did change social environments," Holms said. "Then we scored her . . . Eighty percent of women who have experienced those kinds of changes get sick. That could be anything: the flu, a broken leg, a stomach ulcer. It does not automatically mean a mental problem."

The psychiatric stories published in London followed two particularly unhappy trips on which reporters and photographers hounded Diana and her husband, Prince Charles--their vacation with the royal family over Christmas and a ski trip to Switzerland. When Diana attempted to avoid photographers tracking her on the slopes, the Sun, another tabloid, headlined: "Sulky Di in Photo Fury."

"At one stage . . .," the Sun reported critically of that trip, "she held both her hands over her face for a full five minutes while Prince Charles was begging her to lift her head up." To avoid the press, the newspaper added, Diana hid on the floor of a speeding car. "One cameraman called to her: 'You're just being stupid.' "

The incidents--in which the princess clearly became exasperated--are what fueled stories about her instability, her relationship with Charles and generally, whether, as the Daily Express put it, "the ice princess [is] on the slippery slope." "If she wants to sulk inside a woolly hat," wrote Jean Rook of the Express, "she shouldn't have taken on the Crown."

The torrent of bad publicity was, in its way, the flip side of the adulation Diana received when she and Charles were engaged, then married, and at the birth of their son, William. Although coverage was so aggressively copious that Buckingham Palace repeatedly appealed to the press to ease up (especially after pictures appeared of a pregnant Diana wearing a bikini), criticism of her seemed out of the question.

That began to change late last summer when Diana, evidently tired of spending a long rainy vacation in Scotland with the rest of the royals, petitioned Charles to let her return to London. Reports of her petulance started to circulate, followed by suggestions that the princess was so upset with her life that she was suffering from "anorexia nervosa," a compulsive dieting disease.

Those stories were demolished every time the princess made a public appearance looking elegantly radiant and so were gradually replaced by tales of her frustration with royal life. Today the Star, claiming that at long last it was publishing the truth about the marriage, said:

"Of course she has had occasional rows with her husband. What young bride hasn't? Of course she is sometimes less than delighted at the endless attention of the world's press. What public figure isn't? But to her family and friends, Princess Diana is a happily married young wife and devoted mother who takes her public and private duties very seriously.

"And she does so against overwhelming odds."

Buckingham Palace welcomed the Star's declaration and a spokesman said he hoped other newspapers would follow the example of restraint. Palace sources said it remains to be seen whether British tabloids in a ferocious circulation war will be able to resist, any more than an alcoholic swearing off drink, the temptation to dwell on Diana's behavior--bad, good or indifferent.

According to the palace, which keeps up with such things, readers' letters to the Star and other newspapers have been overwhelmingly critical of the savaging of Diana. So have been the comments on radio talk shows and call-ins. "People may really have had enough," one source said, acknowledging that the odds of self-restraint lasting are small.

Underlying the whole dispute is one of the main issues about Britain's royal family. "What is a princess for?" Suzanne Lowry wrote in the heavyweight London Sunday Times last month.

"Setting aside tedious constitutional and historical arguments about the succession, divine right, stability etc. the best answer seems to be that a princess is for looking at. Neither Diana nor any other member of the royal family has much function when out of sight. Without press coverage, the royal family would be little more than rich, overdressed people in big houses."

In short, Lowry wrote, Diana has to go on "playing the game" by making herself agreeable to the public attention--and the inevitable gossip--that comes with her position. But for everyone involved, the unresolved question remains: How and where do you draw the line