Prime Minister Boris Johnson paid tribute to the “courage and integrity” of former Soviet leader Mikhail Gorbachev.
Mr Gorbachev’s death at the age of 91 has prompted an outpouring of tributes from world leaders.
Many have referred to the timing of his death during the worst period in Russian-Western relations since the end of the Cold War.
In a post on Twitter, Mr Johnson said he was “saddened” to learn of Mr Gorbachev’s death, at a “time of Putin’s aggression in Ukraine”.
“I am saddened to learn of Gorbachev’s passing,” Mr Johnson said.
“I have always admired the courage and integrity he showed in bringing the Cold War to a successful conclusion.
“At a time of Putin’s aggression in Ukraine, his tireless commitment to opening up Soviet society remains an example for all of us.”
Labor leader Sir Keir Starmer said Mr Gorbachev was “one of the great figures” of the last century who will “be remembered forever”.
In a tweet, he said: “One of the great figures of the 20th century, Mikhail Gorbachev’s pursuit of reform paved the way for conflict diplomacy.
“He will forever be remembered as the last leader of the Soviet Union who had the courage and conviction to end the Cold War.”
Irish Prime Minister Micheal Martin said: “I am deeply saddened to learn of the death of Mikhail Gorbachev, one of the most important political figures of the late 20th century.
“At a time when the threat of nuclear destruction to the world was very real, he saw the urgent need for rapprochement with the West and for greater openness and reform – glasnost and perestroika – in the Soviet Union from then.
“His leadership helped end the arms race between East and West, end the Cold War and bring down the iron curtain that had divided Europe since World War II.”
Former US Secretary of State Henry Kissinger said Gorbachev “had done great service” but was “not able to implement all of his visions”, telling BBC Newsnight: “The peoples of Europe from the East and the German people, and in the end the Russian people, owe him a great debt of gratitude for the inspiration, for the courage to present these ideas of freedom.
Mr. Kissinger, after again acknowledging that Mr. Gorbachev was unable to implement his full vision, added: “He will go down in history as a man who started historic transformations that were at the benefit of humanity and the Russian people”.
Former US Secretary of State James A Baker III, who dealt regularly with Gorbachev for years on issues such as the Berlin Wall and NATO, called the former Soviet leader a “giant”.
“History will remember Mikhail Gorbachev as a giant who led his great nation to democracy,” Mr Baker said.
“He played a vital role in bringing the Cold War to a peaceful conclusion through his decision not to use force to hold the empire together,” the US official added. “I found him to be an honest broker and could count on his word despite the internal pressure in Moscow. He misses the free world immensely.
European Commission President Ursula von der Leyen called Gorbachev a “trusted and respected leader”.
“He played a crucial role in ending the cold war and bringing down the iron curtain,” she wrote on Twitter on Tuesday. “It paved the way for a free Europe. This legacy is one we will not forget.
UN Secretary-General Antonio Gueterres described Gorbachev as a “one-of-a-kind statesman”.
He said: “I was deeply saddened to learn of the death of Mikhail Gorbachev, a one-of-a-kind statesman who changed the course of history. He did more than any other individual to bring peaceful end to the cold war.
“The world has lost an imposing global leader, a committed multilateralist and a tireless advocate for peace.”
Gorbachev died Tuesday “following a serious and long illness” in a Moscow hospital.