Rishi Sunak, centre, waves after winning the Conservative Party leadership contest at the Conservative party Headquarters in London, Monday, Oct. 24, 2022. AP
Here is a selection of top reactions from global leaders:
Biden welcomes 'groundbreaking' PM
Even ahead of Sunak's appointment as prime minister by King Charles III -- the monarch's first new government chief since taking over from his mother Elizabeth II -- US President Joe Biden said it was "groundbreaking" that Britain would be led by a person with Indian heritage.
"It's pretty astounding, a groundbreaking milestone and it matters," Biden said at a White House celebration of the Indian Diwali festival celebrated on Monday.
Zelensky wants a strong partnership
Ukrainian President Volodymyr Zelensky wrote on social media that he hoped Sunak would be able "to successfully overcome all the challenges facing British society and the whole world today".
He added that he was "ready to continue strengthening the Ukrainian-British strategic partnership", following months with London as one of Kyiv's most vocal supporters against Russia's February invasion.
'Tackling challenges' with France
France's Emmanuel Macron tweeted that "together we will continue working to tackle the challenges of the moment, including the war in Ukraine and its many consequences for Europe and the world".
Italy is 'eager' to cooperate
From Italy, newly-installed Prime Minister Giorgia Meloni said that she was "eager to cooperate with (Sunak) and his cabinet on the common challenges, standing for our shared values of freedom and democracy."
'Close friends' with Germany
"I look forward to our further cooperation and partnership in NATO and (the) G7 as close friends," German Chancellor Olaf Scholz tweeted.
'Positive EU-UK relations'
Maros Sefcovic, the European Union's point man on post-Brexit ties with Britain, said that "a positive EU-UK relationship is of strategic importance".
He vowed to work towards "a partnership in full respect of our agreements" -- including the contentious issue of British province Northern Ireland's land border with EU-member Ireland.
European Commission chief Ursula von der Leyen said that "in these testing times for our continent, we count on a strong relationship with the UK to defend our common values".
Russia sees no hope for improvement
Kremlin spokesman Dmitry Peskov said that in Moscow, "we do not see any preconditions, grounds, or hope that in the foreseeable future there will be any positive changes" in the relationship between the United Kingdom and Russia.
Nigeria welcomes 'milestones'
Nigerian President Muhammadu Buhari said that with Sunak "as the first Prime Minister of British-Asian descent and the youngest in about 200 years, these milestones will be especially inspiring for young people" in the Commonwealth, the association mostly of former British colonies.
"We seek to deepen our partnership with Britain" to address challenges including terrorism, food insecurity, energy and climate change, Buhari added.
Address global concerns with Canada
Canadian Prime Minister Justin Trudeau said he hoped to strengthen what was already "one of the strongest relationships between any two countries" further by jointly addressing global concerns, such as Russia's invasion of Ukraine, economic uncertainty and climate change.
Trudeau said he also hoped for a "comprehensive, ambitious, and inclusive Canada-United Kingdom Free Trade Agreement".
Special Diwali wishes from Modi
Indian Prime Minister Narendra Modi did not wait for Sunak to be officially appointed, sending congratulations via Twitter on Monday.
"As you become UK PM, I look forward to working closely together on global issues," Modi wrote.
"Special Diwali wishes to the 'living bridge' of UK Indians, as we transform our historic ties into a modern partnership," he added.
Sunak is married to an Indian, Akshata Murty, the daughter of the co-founder of IT giant Infosys.