Baroness Margaret Thatcher has died aged 87 following a stroke.
The former Conservative Prime Minister's spokesman Lord Bell confirmed the news on Monday morning.
He said: "It is with great sadness that Mark and Carol Thatcher announced that their mother Baroness Thatcher died peacefully following a stroke this morning.
"A further statement will be made later."
Prime Minister David Cameron said: "It was with great sadness that l learned of Lady Thatcher’s death. We've lost a great leader, a great prime minister and a great Briton."
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Buckingham Palace said in a statement: "The Queen was sad to hear the news of the death of Baroness Thatcher. Her Majesty will be sending a private message of sympathy to the family."
Thatcher, whose husband Denis died in 2003 at the age of 88, had been in ill health for some time and had been suffering from dementia.
Meryl Streep's Oscar Award-winning portrayal of her in the film The Iron Lady saw her having frequent conversations with her long-dead husband.