Prime Minister Theresa May has paid tribute to the former UN secretary-general Kofi Annan, who has died aged 80, calling him a "great leader".
His foundation announced the death on Saturday following a short illness.
Mrs May said Mr Annan reformed the UN and made a "huge contribution to making the world he has left a better place than the one he was born into".
Former prime minister Tony Blair said he was a "great diplomat, a true statesman and a wonderful colleague".
Mr Anann served as UN chief from 1997 to 2006 and was awarded a Nobel Peace Prize.
The diplomat, who was originally from Ghana, died in hospital in the Swiss city of Bern. He had been living near Geneva for several years before his death.
Former PM Gordon Brown also paid tribute to Mr Annan, who was the first black African to become UN secretary-general.
Mr Blair said: "I'm shocked and distressed to hear the news about Kofi. He was a good friend whom I saw only weeks ago."
He said Mr Annan was "widely respected and will be greatly missed".
Mr Blair's spell in No 10 coincided with Mr Annan's time as secretary-general and the pair clashed over the 2003 invasion of Iraq, by US and British troops - a war which Mr Annan called illegal.
Mr Brown, who is currently the UN special envoy for global education, said it had been "a privilege" to work with Mr Annan.
He said: "Kofi Annan was a leader of leaders, a wonderful humanitarian and the most compassionate and caring of individuals.
"Personally modest and always softly spoken, he was a titan amongst world statesman who saw wrong and righted it and who witnessed evil and always fought it."
Mr Brown added: "Even in his later years he fought against poverty, injustice and war with all the vigour of youth and I had the privilege of working with him in recent times."
Foreign Secretary Jeremy Hunt also paid tribute.
RIP Kofi Annan, statesman and champion of peace. At times when the value of multilateral organisations are under the microscope he showed that moral leadership can make a big difference— Jeremy Hunt (@Jeremy_Hunt) August 18, 2018
Labour leader Jeremy Corbyn and the mayor of London, Sadiq Khan, both praised Mr Annan's humanitarian work and dedication to searching for peaceful resolutions.
Kofi Annan dedicated his life to building a more just and peaceful world.— Jeremy Corbyn (@jeremycorbyn) August 18, 2018
His efforts in support of conflict resolution and human rights will be remembered. He looked for a peaceful path when others looked for war.
My thoughts are with his family and loved ones.
Sad to hear the news of the death of Kofi Annan. As a former UN secretary-general and Nobel Peace Laureate, he dedicated his life to human rights and building bridges between nations. He will always be remembered as a great humanitarian and a tireless advocate for peace. pic.twitter.com/TNplc45boL— Sadiq Khan (@SadiqKhan) August 18, 2018
Mr Annan was awarded the Nobel Peace Prize in 2001 for helping to revitalise the international body, during a period that coincided with the Iraq War and the HIV/Aids pandemic.
He later served as the UN special envoy for Syria, leading efforts to find a peaceful solution to the conflict.
International Development Secretary Penny Mordaunt said her thoughts were with his family and colleagues after hearing about his death, while her department tweeted that he made "a huge difference to the humanitarian landscape".