Margaret Trudeau, 49, is said to have developed a crush on the two young Windsors, who visited Canada last month.
She has never even met them but claimed that she had. She told a Canadian television interviewer last month that she was friends with the princes and had been skiing with them - which was simply not true. And then she rented a house near the British Columbia ski resort where they were staying in order that they would have a convenient place to come and call on her.
The Vancouver Province newspaper quoted a source familiar with Ms Trudeau's predicament: "She is totally focused on the princes. But the Royal Family wouldn't let her anywhere near them."
When the princes left Canada without seeing her she became despondent, and then desperate.
Last Friday, escorted by family and friends, she was interned at a locked psychiatric facility in a Vancouver hospital.
In her day, Margaret Trudeau was Canada's Princess Diana. Beautiful but naive, a Sixties flower child, she married prime minister Pierre Trudeau in 1971 in the country's wedding of the century. Trudeau was 51 - Canada's most eligible bachelor; she was only 22. She bore three sons then hurled herself in 1977 into the wild celebrity orbit, engaging in wild sex-and- drug excess in New York. She and her husband divorced in 1984.
She had a series of well-publicised affairs with Jack Nicholson, Ryan O'Neal, the Rolling Stones guitarist Ron Wood and other well-known men.
But her star fell in the Eighties and she endured a number of illnesses, from anorexia to depression, and manic depression. In an interview in last week's Maclean's magazine she blamed menopause for her deteriorating mental health. "I thought my usefulness was finished," she told the Toronto publication.