Former prime minister Baroness Thatcher's daughter Carol revealed details today of her mother's struggle with dementia.
In her new book, Carol Thatcher said she first noticed her mother's failing memory while the pair had lunch in 2000.
She also disclosed that she had to repeatedly break the "truly awful" news of her father Sir Denis' death to her mother until the information sank in.
In her book, A Swim-On Part in the Goldfish Bowl: A Memoir, she wrote how her mother's "blotting-paper brain" which had always absorbed information got confused between Bosnia and the Falklands during a conversation about the war in the former Yugoslavia.
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.
"The contrast was all the more striking because, until that point, she'd always had a memory like a website."
In the book, serialised in the Mail on Sunday, Ms Thatcher added: "From the fateful day of our lunch, telltale signs that something wasn't quite right began to emerge.
"Whereas previously you would never have had to say anything to her twice, because she'd already filed it away in her formidable memory bank, Mum started asking the same questions over and over again, unaware she was doing so.
"It might be something innocuous - such as 'What time is my car coming?' or 'When am I going to the hairdresser?' - but the fact she needed to repeat them opened a new and frightening chapter in our lives.
"I had to learn to be patient, a quality I admit is in short supply. I also had to learn she had an illness and that it wasn't personal.
"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."
Recalling Sir Denis' death from pancreatic cancer in 2003, she said: "Losing Dad ... was truly awful for Mum, not least because her dementia meant she kept forgetting he was dead.
"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."
The former Conservative prime minister, 82, has kept a low profile in recent years and her doctor advised her to stop making public speeches on health grounds.
But she made a rare return to the limelight when she visited her old Downing Street domain as a guest of Gordon Brown in September last year and attended this year's Wimbledon tennis tournament.
Ms Thatcher said her mother still had a good memory of her time in office at No 10 "as if her dementia had sharpened her powers of long-term recall".
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