The singer was riding in the 1,200-acre grounds of her magnificent £9 million country estate Ashcombe House, accompanied by her personal assistant, when she tumbled from the unfamiliar horse.

Her husband, Guy Ritchie, the film director, and her two children, Rocco, 5, and Lourdes, 8, were at home in Tollard Royal, on the Wiltshire-Dorset border, for the star’s birthday when the accident happened late yesterday afternoon.

Madonna only recently took up riding, apparently hiring out an entire arena for her first lesson at Stag Riding Stables in Richmond, southwest London.

Mr Ritchie drove his wife to Salisbury District Hospital, where she was treated for a series of fractures. As well as her collarbone, she broke bones in one hand and suffered three cracked ribs. She was expected to be released late last night but experts said that her injuries would take at least six weeks to heal.

The accident comes on the eve of a hectic period for the singer, who is preparing for the launch of a new album, Confessions On The Dancefloor, in November, preceded by the release of her first single since 2003. The working title, Defying Gravity, was, perhaps sensibly, abandoned.

It is unclear what impact the accident will have on the release and subsequent promotions and concert dates. The singer has suffered from joint problems, including a bad back and dodgy knees, after more than two decades of pushing her body to the limit on stage.

Ever since she first gyrated into the spotlight in 1982, after signing a recording contract with Sire Records, Madonna Louise Ciccone has developed an unrivalled reputation for stylistic reinvention.

The release of each of her albums has been accompanied by a bewitching switch of image, from street urchin to sex siren to Earth mother, and even a dalliance as a henna-stencilled devotee of kabbalah.

Even by these standards, however, her most recent incarnation as a millionaire facsimile of the English lady of the manor ranks among the most unexpected. She has professed her passion for her new-found life and is more frequently seen sporting jodhpurs and tweeds than the Jean Paul Gaultier garments with which she made her name as the material girl.

The mellowing of her image followed her marriage in 2000 to Ritchie, the director of Lock, Stock and Two Smoking Barrels, with whom she had the second of her two children. Five years later they are very much playing the role of landed gentry, last month appearing in American Vogue espousing the virtues of country living.

Since the Ritchies moved into Cecil Beaton’s former home, they have waged a protracted legal action to prevent walkers from having right of way across their land.

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