Former British prime minister Margaret Thatcher - once known as one of the world's most formidable political minds - has been suffering from dementia for the past seven years, according to her daughter, Carol.

Carol Thatcher tells in her memoirs of how her 82-year-old mother, nicknamed the 'Iron Lady' for her tough reputation, often struggles to remember things and repeats questions.

In a memoir serialised in the Mail on Sunday newspaper, her daughter paints a picture of a very different woman from the political heavyweight who strutted the world stage in the 1980s.

'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,' she wrote.

'On bad days, she could hardly remember the beginning of a sentence by the time she got to the end.'

Thatcher rarely appears in public these days after being advised by doctors in 2002 that she should avoid public speaking following a series of minor strokes.

People suffering with dementia have a significant loss of the mental skills that affect daily life. It is caused by various factors such as strokes, tumours, head injuries and Alzheimer's disease and affects some 700,000 people in Britain, around two-thirds of them women.

Carol Thatcher said the first sign of dementia appeared when her mother was about 75, when she confused the Falklands and Bosnian conflicts in a conversation over lunch. 

'I nearly fell off my chair,' Carol Thatcher wrote, according to excerpts of the book which will be published next month.

'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 per cent cast-iron damage-proof.'

Thatcher took power in Britain in a general election on May 3, 1979, becoming the nation's first and only female prime minister. She left the famous Number 10 Downing Street office after a record-breaking 11 years in power.

At the peak of her powers in the early 1980s, Thatcher's sheer strength of personality made her one of the West's best known figures, famous for 'handbagging' her opponents into backing her, or at least backing down.

Carol Thatcher wrote that one of the most difficult episodes in recent years had been the death of her father Denis Thatcher in 2003, which her mother often forgot had happened.

'Losing Dad was truly awful for Mum, not least because her dementia meant she kept forgetting he was dead,' she wrote. 'I had to keep giving her the bad news over and over again.'--Reuters