The Queen has been saddened by the death of former Prime Minister Baroness Thatcher, Buckingham Palace said today.
The Queen will send her family a private message of sympathy after the Baroness died peacefully this morning following a stroke.
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."
Work and Pensions Secretary Iain Duncan Smith said Baroness Thatcher was "the reason I came into politics".
He said: "Watching her set out to change Britain for the better in 1979 made me believe there was, at last, real purpose and real leadership in politics once again.
"She bestrode the political world like a colossus.
"This is dreadfully sad news and my thoughts and prayers are with her family."
Senior Tory MP David Davis said: "Margaret Thatcher was the greatest of modern British prime ministers, and was central to the huge transformation of the whole world that took place after the fall of the Soviet Union.
"Millions of people in Britain and around the world owe her a debt of gratitude for their freedom and their quality of life, which was made possible by her courageous commitment to the principles of individual freedom and responsibility.
"Her passing is a very sad event and she will be greatly missed."
Liberal Democrat leader Nick Clegg said: "Margaret Thatcher was one of the defining figures in modern British politics.
"Whatever side of the political debate you stand on, no-one can deny that as prime minister she left a unique and lasting imprint on the country she served.
"She may have divided opinion during her time in politics but everyone will be united today in acknowledging the strength of her personality and the radicalism of her politics.
"My thoughts are with her family and friends."
On his Twitter feed, London Mayor Boris Johnson said: "Very sad to hear of death of Baroness Thatcher. Her memory will live long after the world has forgotten the grey suits of today's politics."