After winning the campaign to be the next Prime Minister, Rishi Sunak issued a call for unity in the face of a "deep economic crisis."
He won the Tory leadership election when competitor Penny Mordaunt failed to obtain enough MP support.
Mr. Sunak stated in his inaugural address that bringing his party and the UK together would be his "utmost priority."
Mr. Sunak will be the first British Asian prime minister in more than 200 years and the youngest.
Mr. Sunak, a 42-year-old Hindu, is scheduled to assume office on Tuesday once the King formally appoints him.
Following her final cabinet meeting at 9:00 a.m. BST, the outgoing prime minister will make a speech outside No. 10 before traveling to Buckingham Palace for her farewell visit with the King.
Following that, Mr. Sunak will have his first encounter with the queen, at which he will be invited to form a government.
He will proceed to Downing Street at about 11:35 a.m. to make a statement before entering No. 10.
According to the White House Press Secretary, President Joe Biden planned to phone Mr. Sunak after meeting with the King to "give his congratulations."
Mr. Sunak becomes the third Conservative prime minister since the party's victory in the 2019 general election, forcing Labour to lead calls for an early election as a result.
Mr. Sunak informed them in his address, which took place behind closed doors, that this would be a difficult moment and ruled out an early general election, according to MP Simon Hoare.
The former chancellor claimed the Conservative Party, which is trailing Labour in the polls by large majorities, is facing an "existential threat," but that by unifying, it can win the next election.
Later, in a two-minute TV statement, Mr. Sunak pledged to serve with "integrity" and praised departing Prime Minister Liz Truss for steering the country through "exceptionally tough conditions."
"The United Kingdom is a wonderful nation, but there is no question that we are facing a significant economic problem," he remarked. "We now require stability and unity, and I will make it my top goal to unite our party and our nation."
Mr. Sunak's election as Tory leader effectively concludes the former chancellor's swift political return after losing the last leadership race to Ms. Truss during the summer.
Ms. Truss, whose tax-cutting program Mr. Sunak derided as "fairytale economics," congratulated him and pledged her "unconditional support."
Her replacement will assume office at a time of economic crisis and fiscal strain, exacerbated by Ms. Truss's mini-budget, the majority of which has been canceled.
After a period of heavy political turbulence under Ms. Truss's government, Conservative Party Chairman Jake Berry said it was time for the party to "rally foursquare behind Rishi."
Penny Mordaunt, Mr. Sunak's leading competitor, withdrew from the latest election minutes before the decision was revealed, acknowledging that "colleagues agree we need clarity today."
"This is a historic decision that demonstrates the variety and brilliance of our party," Ms. Mordaunt added. "Rishi has my whole backing."
Ms. Mordaunt pushed the leadership battle to the line, but she was under pressure to rally around Mr. Sunak after former Prime Minister Boris Johnson dropped out on Sunday.
Mr. Johnson, who resigned as Prime Minister barely seven weeks ago, said he had enough support to run but that now was "not the appropriate moment."
According to sources, Mr. Sunak and Mr. Johnson met on Saturday as Tory MPs debated who to support in the second Tory leadership election in four months.
Mr. Sunak was chancellor in Mr. Johnson's cabinet and had to shepherd the UK economy through the Covid-19 epidemic within weeks of gaining the job.
However, he resigned as chancellor in July when the administration was besieged by scandals, which fueled a cabinet revolt that threw Mr. Johnson out of power.
Mr. Sunak rose to the top of British politics after being elected as an MP for the North Yorkshire district of Richmond in 2015.
Mr. Sunak worked in banking before becoming an MP and is regarded to be one of the wealthiest MPs.
However, his family's finances came under sharp scrutiny earlier this year when his wife, Akshata Murty, had tax issues exposed.