Baroness Thatcher has been suffering from dementia for at least seven years, her daughter has revealed in the first public account of the former Prime Minister’s illness.

Carol Thatcher tells how her mother’s memory is failing and says that she sometimes struggles to finish sentences but has flashes of her “old self . . . particularly when events from the past \ mentioned”.

Ms Thatcher details her mother’s fading mental powers in a memoir of life in and out of Downing Street. The death of Sir Denis Thatcher in 2003 was “truly awful” since she “kept forgetting he was dead”, according to the book, A Swim-on Part in the Goldfish Bowl: A Memoir. “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.”

Lady Thatcher, 82, was advised to stop public speaking on health grounds shortly before her husband’s death. Although it is known widely that she has suffered a number of strokes, her dementia has been far less publicised. The first signs were apparent when she was 75, according to her daughter, who tells of her becoming confused about the Bosnian and Falklands conflicts during a lunch in 2000.

“I almost fell off my chair. Watching her struggle with her words and her memory, I couldn’t believe it. I had always thought of her as ageless, timeless and 100 per cent cast-iron damage-proof. The contrast was all the more striking because she’d always had a memory like a website.”

In the book, serialised in The Mail on Sunday, Ms Thatcher adds: “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 frightening chapter in our lives. I had to learn to be patient. 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.”

On bad days Lady Thatcher can “hardly remember the beginning of a sentence by the time she got to the end”. Ms Thatcher adds: “It took us time to realise that couldn’t remember a newspaper headline she had just read, or what she’d had for breakfast that morning. But when a friend asked, off the cuff, ‘Oh Margaret, do you remember rationing?’ he got a full ten minutes of my mother’s best grocer’s daughter’s tips on how to jazz up tinned Spam or powdered egg.”

Ms Thatcher, a journalist, acknowledges that her experience will be recognised by many families and says that her mother is relatively well placed. Lady Thatcher has a driver, a police guard, a personal assistant and “other loyal devotees”.Although she mostly keeps a low profile, she occasionally returns to the limelight to prove her iconic status and ability to cause controversy. She was on the cover of Vogue this summer and last September embarrassed David Cameron by returning to Downing Street as the guest of Gordon Brown.

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