Princess Margaret, the high-spirited and unconventional sister of Queen Elizabeth II, died Saturday after a life that echoed with regret and thwarted love. She was 71.

The princess died peacefully in her sleep at King Edward VII Hospital at 6:30 a.m., a statement from Buckingham Palace said.

Margaret suffered a stroke Friday afternoon and developed cardiac problems during the night. She was taken from Kensington Palace to the hospital at 2:30 a.m., the statement said.

"Lord Linley and Lady Sarah were with her and the Queen was kept fully informed throughout the night," the statement said.

A heavy smoker for many years, she suffered repeated respiratory illnesses and had part of a lung removed in January 1985. She had a mild stroke in February 1998 and another in March 2001.

Margaret was last seen in public before Christmas at Princess Alice, the Dowager Duchess of Gloucester's 100th birthday party. Margaret was confined to a wheelchair and wore heavy dark glasses, her sight having been affected by a stroke.

The queen left Sandringham, her Norfolk estate, Friday and traveled to Windsor where she remained in touch with developments. The 101-year-old Queen Mother Elizabeth, who is recovering from a persistent cold, stayed on at Sandringham.

Speaking on his plane on the way to Sierra Leone Saturday, Prime Minister Tony Blair expressed his sadness at the death.

"I'm deeply saddened to hear of the death of Princess Margaret. My thoughts are with the queen, Queen Elizabeth the Queen Mother and the rest of the royal family at this time," Blair said.

The royal death will cast a shadow over this year's Golden Jubilee celebrations, marking the 50th anniversary of the queen's accession to the throne. A nationwide tour and a full program of Jubilee celebrations is planned for later in the year. On Feb. 18, the queen is due to start a visit to Jamaica, New Zealand and Australia.

In the 1950s, Margaret's ill-starred romance with royal aide Peter Townsend made headlines around the world because he was divorced. Twenty-three years later, she became a divorcee herself — the first in the queen's immediate family — when her marriage to the photographer Lord Snowdon was dissolved.

Margaret had not remarried.

Despite the upheavals, the publicity and the differences in their personalities, the princess and her dignified sister remained close.

"In our family," Margaret once said, "we don't have rifts. We have a jolly good row and then it's all over. And I've only twice ever had a row with my sister." She didn't say what they argued about.

Margaret's lighthearted and informal behavior was sometimes offset by an unsettling "royal" streak.

Even her close friends had to call her "Ma'am," although members of the family were said to have gotten away with "Margot." If any of her companions crossed the line of familiarity, they risked her famous icy, blue-eyed "acid drop" stare.

She once explained it as "a defense mechanism. I'm not aware that I'm doing it."

It may seem ironic that a princess whose first love foundered on the taboo of divorce became the first senior royal to dissolve her marriage.

But it is entirely in keeping for a woman who often strained convention and attracted press criticism.

The high-spirited, party-going princess first ran afoul of royal protocol when she was 22, shortly after sister took the throne in 1952.

Margaret fell in love with Group Capt. Townsend, a hero of the Battle of Britain and a former aide to her father, King George VI.

The shock of King Edward VIII's abdication to marry a divorcee was still fresh in the public memory, and the Church of England forbade remarriage of a divorced person. The government firmly opposed such a marriage for the sister of the queen, who was temporal head of the state church.

After more than two years of negotiation, press speculation and enforced separation from Townsend, Margaret announced in October 1955 that she would not marry him, "mindful of the church's teachings that Christian marriage is indissoluble, and conscious of my duty to the Commonwealth."

Townsend, who remarried happily, reflected on his romance with Margaret in a 1978 autobiography: "I simply hadn't the weight, I knew it, to counterbalance all she would have lost. It was too much to ask of her, too much for her to give."

Margaret blamed the queen's private secretary, Sir Alan Lascelles, for campaigning against the match, and never forgave him. He retired to an apartment at Kensington Palace, where Margaret lived, and she was heard to say when he walked by, "There goes the man who ruined my life."

Margaret grew up in the peculiarly isolated world of royal children, surrounded by adults, doted on by the public, and strictly brought up.

When Margaret and Elizabeth were born, their father was the Duke of York, second son of the king. Although the public was fascinated by "the little princesses," the girls lived largely outside the public eye.

The abdication of their uncle thrust their father onto the throne and set Elizabeth on the path to monarchy.

Margaret, only 6, told her sister: "Does that mean you're going to be queen? ... Poor you."

Vivacious, flirtatious and a bit of a comedian, Margaret Rose was an outgoing child quite different from her tidy, sensible and serious-minded sister.

An elderly courtier who collided with little Margaret Rose when she was cartwheeling down a Buckingham Palace corridor was said to have sighed, "Thank God the other one was born first."

Margaret was musical, liked to perform and had a gift for mimicry. Unlike most of the other royals, who prefer tweedy, outdoor pursuits, she supported the arts and loved opera, theater and dance. She was often seen at restaurants and nightclubs with groups of friends and smoked her ever-present cigarettes in a long, distinctive cigarette holder.

Legendary jazzman Louis Armstrong, following a conversation with Margaret about music, told the press, "Your Princess Margaret is one hip chick."

In 1958, she began to see Antony Armstrong-Jones, whose career as a society photographer was flourishing. She was 30 when they were married in Westminster Abbey on May 6, 1960.

Their first child, David, was born on Nov. 3, 1961. Lady Sarah was born May 1, 1964.

By the early 1970s, the marriage was beset by rumors of infidelity and the two went their separate ways.

In 1973, the princess, then 43, met Roderic "Roddy" Llewellyn, a man of no apparent means 17 years her junior.

They were romantically linked for half a dozen years, and the publication of a photograph of them together on the Caribbean island of Mustique was followed by announcement of the Snowdons' formal separation.

The marriage was dissolved in May 1978, with little fuss from a public that expressed nothing but sympathy and regret for a woman who seemed never to have found lasting love.