Princess Margaret, whose marriage fell apart years ago, will divorce Lord Snowdon, father of their two teen-age children. The divorce will be the first in the immediate royal family since Henry VIII, four centuries ago.

The decision, announced yesterday by a spokesman at Kesington Palace, will cost Margaret, 47, nothing but a bad press. She will keep her title and children, remain sixth in line to her sister's throne, hang on to her $100,000-a-year allowance and stay rent-free at Kensington Palace.

Moreover, the spokesman said, she can continue her friendship with Roderick ("Roddy") Llewellyn, the 31-year-old aspiring pop singer and, currently, the other man in the affair.

The grounds for the divorce, the spokesman said, are simple: "The marriage has broken down and the couple have lived apart for two years."

Under British law, no other specific cause need be cited. After a couple formally separates for two years, satisfies a court registrar that they have lawfully divided their property and arranges for their children, a divorce is automatically granted when neither objects.

Margaret is presently hospitalized down with gastro-enteritis. Snowdon, a distinguished photographer and noted jet-setter, would not comment. Llewellyn, a sometimes landscape gradner and margaret's constant attendant for the past five years, was unreachable, on vacation in Tangier, Morocco.

So once again Margaret's tangled and heavily publicized private life is titillating the British. They always enjoy a whispered bit of royal scandal but simultaneously insist on a Victorian standard in public.

Monarchy and divorce have been inextricably bound up with each other here since the birth of the Church of England.

Henry VIII used the pope's refusal to grant him a divorce from Catherine of Aragon as his pretext to seize Catholic church lands and establish his own ecclesiatical hierarchy. To underlin his power, he also divorced his fourth wife, Anne of Cleves, in 1939.

Margaret's sister, Queen Elizabeth II, is supreme governor of the established church and this role has made it politically and theologically difficult for Royals to divorce. Until recenty, the Church of England resolutely set its face against the practice. Even today, only a handful of its clergymen will remarry a divorced person inside the church.

Its spiritual head, the archbishop of Canterbury, played a key role in persuading Margaret's uncle Edward, to marry American divorcee Wallis give up his throne when he sought to Simpson.

Only 23 years ago, Margaret was confronted with a choice between her first love, dashing but divorced Group Capt. Peter Townsend, and her title. She chose the title and the perquisites that go with it.

Margaret then moved a nation by declaring that she was "mindful of the church's teaching that Christian marriage is indissoluble" and her "duty to the Commonwealth."

Since then, the church has taken a more relaxed attitude. Most clerics now assert that the indissolubility of marriage must be balanced with tolerance, understanding and compassion for those who make genuine mistakes.

If few clerics will remarry a divorced person, nearly all will offer a special service of blessing for those who wed a second time.

The widespread public hostility to Margaret here does not merely reflect a fundamentalist religious view. The fact is that the British monarchy, unlike those in Scandinavia and elsewhere in Western Europe, insists on perpetuating a mystique, a claim that the Royals are somehow above the common herd.

British royalty stays apart from the mass of citizens, greeting them only under highly formalized circumstances. Even Margaret's modern-minded nephew and heir to the throne, Charles, has said that a demystified monarchy would not suit Britons.

Whether it would or not, the mystical monarchy suits Margaret and her relatives. Collectively, they enjoy an allowance from parliament of $5.2 million but this is only for expenses. The queen and her family "own" huge estates, stocks, paintings and much more - all of it tax-free.

Since the queen reigns but does not rule, she and Margaret are a kind of superior civil servant. And since taxpayers ultimately pay the freight, many feel they are entitled to demand a standard of private behavior they would not impose on themselves.

After she gave up Townsend, Margaret seemed happy with Snowdown and their two children, Lady Sarah Armstrong-Jones, now 14, and Viscount Linley, 16. Like Margaret, Snowdon enjoys jazz, discotheques, eating and drinking.

Snowdon however, is a genuinely talented photographer whose pictures of the aged and impoverished particularly have won high critical praise.

Margaret's taste runs to Frank Singtra ballads.

Two years ago, Margaret and Snowdon separated legally. Her affair with Llewellyn, whom she met on a blind date at a country house weekend, had already been photographed frequently by press photographers other than her husband.

The palace spokesman said Margaret has no plans to remarry but will, if she wants, see Llewellyn "from time to time." Roddy has said he isn't the marrying kind anyway. "I'm too selfish," he once told The Guardian.

The divorce is almost certain to bring fresh attacks in Parliament on the size of the royal allowances. Just a month ago, the Commons voted the family a 9.2 percent increase to match the inflation here.

The first assault came last night from John Lee, a Labor Member of Parliament. He insisted - contrary to what legal authorities say is the fact - that the divorce "must have a direct bearing upon the constitutional position of the princess and of the position of the royal family generally."

Then came the meat.

"It must make more urgent the need for a review of the scale of royal remuneration for duties performed," Lee said.