Lady Margaret Thatcher, once hailed as the iron butterfly for her determined crusade against communism, is now suffering from dementia, her daughter says.
She struggles to finish her sentences, does not know where she lives and even forgets that her husband Denis is dead.
In a memoir to be published next month, Carol Thatcher paints a picture of "the new Lady T," a much-diminished figure created by the progressive effects of dementia and a series of minor strokes.
Lady Thatcher, now 82, first started to show signs of mental deterioration almost a decade ago, her daughter recalls.
In her book, serialised in a Sunday newspaper, Miss Thatcher wrote how her mother confused Bosnia and the Falklands during a conversation about the war in the former Yugoslavia.
And it was the Falklands war against a strutting but weak Argentina that sealed her reputation as a tough leader and she went on to win the national elections in 1983 by a landslide.
She wrote: "I almost fell off my chair. Watching her struggle with her words and her memory, I couldn't believe it. She was in her 75th year but I had always thought of her as ageless, timeless and 100% cast-iron damage-proof."
Miss Thatcher describes the gradual decline in her mother's faculties, leaving her a shadow of her formidable earlier self.
The "blotting paper memory" that allowed her to absorb huge volumes of information in office is now failing badly, and her ability to understand and interact with the world around her is fading.