The Queen has been saddened by the death of former Prime Minister Baroness Thatcher, Buckingham Palace said today.
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."
Barack Obama has led tributes from around the globe to Baroness Thatcher, saying the "world has lost one of the great champions of freedom and liberty, and America has lost a true friend".
The US president joined world leaders past and present in paying tribute to the former prime minister.
He said in a statement: "As a grocer's daughter who rose to become Britain's first female prime minister, she stands as an example to our daughters that there is no glass ceiling that can't be shattered.
"As prime minister, she helped restore the confidence and pride that has always been the hallmark of Britain at its best.
"And as an unapologetic supporter of our transatlantic alliance, she knew that with strength and resolve we could win the Cold War and extend freedom's promise."
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."
Former SDLP leader John Hume described Baroness Thatcher as an "extremely divisive figure".
"Events in Ireland dominated and defined Margaret Thatcher's time as prime minister," he said.
"Her hard-line, belligerent and uncompromising approach during the hunger strikes won her few friends among nationalists.
"There is no doubt that her actions caused great hurt and harm. As a result she remained an extremely divisive figure and we clashed politically on many occasions over our differing views on how to achieve a peaceful solution to the situation in the north."
Ian Paisley Snr, said the ex-prime minister was a "great".
"Our country has become much the poorer for the passing of Baroness Thatcher," he said.
"In every phase of life she was great - great as a woman, great as a wife, great as a mother, great as a political candidate, great as a member of Parliament, especially as the first woman prime minister, great as a winner of the war, and great as a member of the House of Lords."
Sinn Fein president Gerry Adams said Margaret Thatcher did "great hurt to the Irish and British people during her time as British prime minister".
"Working class communities were devastated in Britain because of her policies.
"Her role in international affairs was equally belligerent whether in support of the Chilean dictator Pinochet, her opposition to sanctions against Apartheid South Africa; and her support for the Khmer Rouge.
"Here in Ireland her espousal of old draconian militaristic policies prolonged the war and caused great suffering."
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."
UK Independence Party leader Nigel Farage wrote on Twitter: "Very sad to hear of the death of Margaret Thatcher, a great patriotic lady."
Former employment minister Tony McNulty wrote: "Former Conservative Prime Minister Baroness Thatcher has died following a stroke. God bless her and thoughts are with her family. RIP."
Andrew Selous, Tory MP for South West Bedfordshire, wrote: "Very sorry to learn of Lady Thatcher's death."
Lord Sugar became one of the first to express his condolences, writing: "Margaret Thatcher died today. A great lady she changed the face of British politics, created opportunity for anyone to succeed in the UK. RIP."
In a further posting, he added: "Baroness Thatcher in the 80s kick-started the entrepreneurial revolution that allowed chirpy chappies to succeed and not just the elite."
Former Conservative prime minister Sir John Major described Baroness Thatcher as a "true force of nature" and a "political phenomenon".
He said: "In government, the UK was turned around under - and in large measure because of - her leadership.
"Her reforms of the economy, trades union law, and her recovery of the Falkland Islands elevated her above normal politics, and may not have been achieved under any other leader.
"Her outstanding characteristics will always be remembered by those who worked closely with her: courage and determination in politics, and humanity and generosity of spirit in private."
Baroness Thatcher's former Cabinet minister Norman Tebbit told the BBC: "It's immensely sad that we have lost one of the greatest political figures of the second half of the 20th century.
"I was proud to have served in her government and to have worked with her in those years.
"We could do with another one like her right now."
Speaking on behalf of the Catholic Bishops' Conference, the Archbishop of Westminster, the Most Rev Vincent Nichols, said: "It was with sadness that we heard the news of the death of Baroness Thatcher, who served this country for many years both as a Member of Parliament and as Prime Minster.
"We pray for the repose of her soul and for the intentions of her family and all those who now mourn for her."
Labour former home secretary David Blunkett said: "Margaret Thatcher was a most formidable opponent, undoubtedly an outstanding leader and, as the first woman prime minister in the United Kingdom, a groundbreaking politician.
"I have to acknowledge her deep commitment to her own values and her determination, although, with Bernard Ingham at her side, she was the first modern exponent of carefully worked spin, which allowed her to present compromise as merely delay, and deep irritation with opponents in her own party as principled stance."
Former prime minister Gordon Brown said: "Sarah and I have sent messages to Lady Thatcher's son Mark and daughter Carol, offering our condolences to them and to the Thatcher family and commemorating Lady Thatcher's many decades of service to our country.
"She will be remembered not only for being Britain's first female Prime Minister and holding the office for 11 years, but also for the determination and resilience with which she carried out all her duties throughout her public life. Even those who disagreed with her never doubted the strength of her convictions and her unwavering belief in Britain's destiny in the world."
Scotland's First Minister Alex Salmond said: "Margaret Thatcher was a truly formidable prime minister whose policies defined a political generation.
"No doubt there will now be a renewed debate about the impact of that legacy.
"Today, however, the proper reaction should be respect and condolences to her family."
Scottish Conservative leader Ruth Davidson said: "Baroness Thatcher was a true Conservative revolutionary - challenging outdated institutions, confronting vested interests and transforming Britain into a property-owning democracy."
Former Number 10 spin chief Alastair Campbell wrote on Twitter: "disagreed with much she did but Mrs Thatcher was one hell of a PM to cover as a journalist, and one of small number of real change PMs."
Commons Speaker John Bercow said: "I was deeply saddened to learn of the death of Baroness Thatcher. She was a distinguished Parliamentarian and a formidable prime minister. My thoughts are with her family during this difficult time."
Irish Taoiseach Enda Kenny described Mrs Thatcher as a formidable leader.
"Mrs Thatcher was a formidable political leader who had a significant impact on British, European and world politics. During her 11 years as prime minister, she defined an era in British public life," he said.
"While her period of office came at a challenging time for British-Irish relations, when the violent conflict in Northern Ireland was at its peak, Mrs Thatcher signed the Anglo-Irish Agreement which laid the foundation for improved North-South cooperation and ultimately the Good Friday Agreement.
"I extend my deepest sympathies to her family and the Prime Minister David Cameron."
Foreign Secretary and former Tory leader William Hague said Mrs Thatcher "rescued the country in the 1980s".
He told Sky News: "She gave the country hope in so many ways and also gave great hope to millions of people around the world, particularly those behind the Iron Curtain, that they could be free."
Home Secretary Theresa May said: "I was very sad to hear of Baroness Thatcher's death, and my thoughts are with her family at this time.
"Margaret Thatcher was one of the giants of the 20th century and one of our greatest prime ministers.
"As the first woman to reach that office, she remains an inspiration to millions of women of all political persuasions. Her considerable legacy continues to shape British politics to this day."
President of Ireland Michael D Higgins said Mrs Thatcher's place in history has been secured.
"She will be remembered as one of the most conviction-driven British Prime Ministers who drew on a scholarship that demanded markets without regulation," Mr Higgins said.
"The policies of Mrs Thatcher's government in regard to Northern Ireland gave rise to considerable debate at the time.
"However, her key role in signing the Anglo-Irish Agreement will be recalled as a valuable early contribution to the search for peace and political stability."
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