London - Former British prime minister Margaret Thatcher, 82, is suffering from dementia, her daughter Carol reveals in a new book, excerpts of which were published by a British newspaper Sunday.

Carol Thatcher writes that she first noticed slips in Baroness Thatcher's legendary memory during a lunch date in 2000, when her mother, prime minister from 1979 to 1990, confused the Falkland and Bosnian wars.

"I almost fell off my chair. Watching her struggle with her words and her memory, I couldn't believe it," Carol Thatcher, a journalist, wrote in the Mail on Sunday.

"She was in her 75th year but I had always thought of her as ageless, timeless and 100-per-cent cast-iron damage-proof," she said.

Carol Thatcher tells how she had repeatedly to tell her mother of the death of her husband, Sir Denis, in June 2003 before the news sank in.

The lunch in 2000 marked the start of a series of "telltale signs that something wasn't quite right," the journalist said.

But she also said the former prime minister's memory of her time at No 10 Downing Street was still excellent, as if her dementia had sharpened her powers of long-term recall."

The book, A Swim-On Part in the Goldfish Bowl: A Memoir, is being serialized in the Mail on Sunday.

Thatcher, famous for taking on the British unions head-on and for implementing market reforms, has avoided public engagement on the advice of her doctors in recent years.