Former Soviet president Mikhail Gorbachev has died aged 91.
Mr Gorbachev is best known for opening up the USSR and for bringing the Cold War to a peaceful end, but he was unable to prevent his country collapsing in 1991.
He died on Tuesday after a long illness, a statement from the Central Clinical Hospital said. No other details were given.
Tributes have flowed from world leaders and politicians following his death.
Prime Minister Boris Johnson said: “In a time of Putin’s aggression in Ukraine, his tireless commitment to opening up Soviet society remains an example to us all.”
Mikhail Gorbachev - In pictures
Speaking outside a police station in Lewisham on Wednesday morning, Mr Johnson said the Kremlin is “intent on undoing the good” of Mr Gorbachev and attempting to “recreate that Soviet empire” with its war in Ukraine.
The Prime Minister said: “Mikhail Gorbachev is one of those people who changed the world and unquestionably changed it for the better. When you look at what he did to make Europe whole, free, to give freedom to the countries of the former Soviet Union - it was quite an extraordinary thing.
I'm saddened to hear of the death of Gorbachev.
I always admired the courage & integrity he showed in bringing the Cold War to a peaceful conclusion.
In a time of Putin’s aggression in Ukraine, his tireless commitment to opening up Soviet society remains an example to us all.
— Boris Johnson (@BorisJohnson) August 30, 2022
“And of course, Mikhail Gorbachev is one of those people who triggered a change, a series of changes, that perhaps he didn’t anticipate.
“Maybe he paid his own political price for it but when history is written, he will be, I think, one of the authors of fantastic change for the better in the world.
“And what I worry about today is that the current leadership in Moscow is intent on undoing the good of Mikhail Gorbachev, and is intent on a revanchist attempt, a revenge driven attempt, to recreate that Soviet empire, and you’re seeing that in Ukraine - that’s the tragedy, something that Mikhail Gorbachev would have thought was absolutely unthinkable, unwarranted.”
Labour leader Sir Keir Starmer said: “One of the great figures of the 20th Century, Mikhail Gorbachev’s pursuit of reform forged a path for diplomacy over conflict.”
US President Joe Biden called Mr Gorbachev a “man of remarkable vision” and a “rare leader” who had “the imagination to see that a different future was possible and the courage to risk his entire career to achieve it.
“The result was a safer world and greater freedom for millions of people,” Mr Biden said in a statement.
President of the EU Commission, Ursula von der Leyen, said Mr Gorbachev was a “a trusted and respected leader”.
“He played a crucial role to end the Cold War and bring down the Iron Curtain. It opened the way for a free Europe. This legacy is one we will not forget,” she wrote on Twitter.
French President Emmanuel Macron said: “My condolences for the death of Mikhail Gorbachev, a man of peace whose choices opened up a path of liberty for Russians. His commitment to peace in Europe changed our shared history.”
Russian President Vladimir Putin expressed “his deepest condolences,”, his spokesman Dmitry Peskov told Interfax news agency. “Tomorrow he will send a telegram of condolences to his family and friends.”
Taoiseach Micheal Martin said Mikhail Gorbachev had "changed the world" and was “one of the most significant political figures of the late 20th century.”
British Chancellor Nadhim Zahawi said his death provides “a moment to reflect on the importance of leadership in world affairs”.
Actor and former Governor of California Arnold Schwarzenegger said: “Mikhail Gorbachev was one of my heroes, and it was an honor and a joy to meet him. I was unbelievably lucky to call him a friend.
"All of us can learn from his fantastic life."
Though in power less than seven years, Gorbachev, who came into office in 1985, made a series of significant changes. But they resulted in the collapse of the authoritarian Soviet state, the freeing of Eastern European nations from Russian domination and the end of decades of East-West nuclear confrontation.
He spent his last months in office watching republic after republic declare independence until he resigned on December 25, 1991. The Soviet Union ended a day later.
A quarter-century after the collapse, Gorbachev told The Associated Press that he had not considered using widespread force to try to keep the USSR together because he feared chaos in a nuclear country.
“The country was loaded to the brim with weapons. And it would have immediately pushed the country into a civil war,” he said.
Margaret Thatcher quickly emerged as his most passionate Western supporter and champion of his efforts for reform. He was a man she admired, an accolade rarely bestowed by the then prime minister.
It was when he visited Britain in 1984, four months before he assumed power, that she said: “I like Mr Gorbachev. We can do business together.” His new style, she said, had “brought hope to the whole world”.
But many of the changes, including the Soviet breakup, bore no resemblance to the transformation that Gorbachev had envisioned when he became the Soviet leader in March 1985.
By the end of his rule he was powerless to halt the whirlwind he had sown. Yet Gorbachev may have had a greater impact on the second half of the 20th century than any other political figure.
“I see myself as a man who started the reforms that were necessary for the country and for Europe and the world,” Gorbachev told The AP in a 1992 interview shortly after he left office.
“I am often asked, would I have started it all again if I had to repeat it? Yes, indeed. And with more persistence and determination,” he said.
Mr Gorbachev won the 1990 Nobel Peace Prize for his role in ending the Cold War and spent his later years collecting accolades and awards from all corners of the world.
He was widely despised at home. Russians blamed him for the 1991 implosion of the Soviet Union — a once-fearsome superpower whose territory fractured into 15 separate nations. His former allies deserted him and made him a scapegoat for the country’s troubles.
The official news agency Tass reported that Mr Gorbachev will be buried at Moscow’s Novodevichy cemetery next to his wife.
Kremlin spokesman Dmitry Peskov said in a statement carried by Russian news agencies that President Vladimir Putin offered deep condolences over Mr Gorbachev's death and would send an official telegram to his family.