A personal assistant to actress and princess of Monaco Grace Kelly died from “unpredictable and extremely rare complications” after falling outside her Maida Vale home, a court heard.

Phyllis Earl, 77, fractured her hip when she fell over the wire charging her beloved G-Wiz electric car in Randolph Avenue on December 6.

She went to St Mary’s Hospital in Paddington but died the next day after suffering from the rare “bone cement implantation syndrome” during routine hip replacement surgery.

The much-loved grandmother lived in the Palace of Monaco alongside Princess Grace and Prince Rainier for four years between 1958 and 1962 when she married Julian Earl, the great-nephew of English author William Somerset Maugham.

The two families formed a long-lasting friendship with Prince Albert of Monaco, the ring bearer at the Earls’ wedding, and Princess Caroline, a flower girl.

“Princess Grace was only a couple of years older than my mother and she had the most wonderful time working for her,” said Mrs Earl’s daughter Cordelia Uys. “She absolutely adored her.”

Mrs Earl, who later in life ran a property business and volunteered extensively for charity, appeared in a feature on the American news channel CNN in 2008 under the headline Granny Loves Her G-Wiz.

Westminster Coroner’s Court yesterday heard that Mrs Earl, who had been a widow since 1990, went into sudden cardiac arrest after surgery had “initially proceeded uneventfully”.

But after waiting for the bone cement to set for six minutes, Mrs Earl began experiencing a severe reaction.

Orthopaedic consultant Peter Reilly, who supervised the procedure, told the court he had conducted “hundreds and hundreds” of similar operations and had never seen such an “unpredictable and untreatable event” in the circumstances.

When asked if there were any concerns prior to the operation or whether anything could have been done differently, Mr Reilly answered “no” to both questions.

Mrs Earl had suffered from the “poorly understood, very rare complication” known as “bone cement implantation syndrome”, the court heard.

Coroner Dr Fiona Wilcox said: “Despite the presence of an experienced team and more than an hour of attempted resuscitation, her life could not be saved.”

Speaking outside the court, Mrs Uys said: “It was a total tragedy that such an incredibly rare thing should happen to our mother.”

The charity Survival International, who Mrs Earl spent 10 years volunteering for, has established the Phyllis Earl Internship in her memory.