Lord Bell said: "It is with great sadness that Mark and Carol Thatcher announced that their mother Baroness Thatcher died peacefully following a stroke this morning."
Baroness Thatcher was Conservative prime minister from 1979 to 1990.
She was the first woman to hold the post. Her family is expected to make a further statement later.
Baroness Thatcher, born Margaret Roberts, became the Conservative MP for Finchley, north London in 1959, retiring from the Commons in 1992.
Having been education secretary, she successfully challenged former prime minister Edward Heath for her party's leadership in 1975.
She won general elections in 1979, 1983 and 1987.