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.
"The woman who had dominated discussions for so long could no longer lead debates or keep up with the thread of a drinks-party conversation. On bad days, she could hardly remember the beginning of a sentence by the time she got to the end," Miss Thatcher writes.
"On any visit, I'd pop into the kitchen to scan the noticeboard where her calendar was written out, before going to see her in the sitting room.
'Hi, Mum,' I'd say, 'how was your lunch with so-and-so in the House of Lords?' She'd look up as if she barely recognised me and say: 'Did I, darling?'
Perhaps most poignant is Miss Thatcher's revelation that her mother often forgets the death of Sir Denis. He died of cancer in 2003.
"Losing Dad ... was truly awful for Mum, not least because her dementia meant she kept forgetting he was dead," Miss Thatcher writes.
"I had to keep giving her the bad news over and over again.
"Every time it finally sank in that she had lost her husband of more than 50 years, she'd look at me sadly and say 'Oh', as I struggled to compose myself. 'Were we all there?' she'd ask softly."
Miss Thatcher adds: "That's the worst thing about dementia: it gets you every time.
"Sufferers look and act the same but beneath the familiar exterior something quite different is going on. They're in another world and you cannot enter."
Lady Thatcher has made few public appearances in recent years. Last year, Gordon Brown invited her back to No 10 Downing Street for a high-profile visit that many saw as an attempt to woo Conservative voters.
Although she lives in her London home, her daughter reveals that the former premier now thinks more often of her childhood home in Lincolnshire.
"My mother hadn't mentioned her birthplace, Grantham, to me for many years until it started cropping up in sentences and I realised she now thought of it as her 'home'."
And many are outraged that the carefully crafted public image of the 'great' lady should be coming unstuck at the hands of her own daughter, even if it is all depredations of time.
Incidentally Ronal Reagan, her close comrade in arms and the man adored universally for decisive conquest over communism, also fell a victim to dementia.