Rishi Sunak will become Britain's third prime minister in three months, and the first of Indian heritage after winning the Conservative Party's leadership.
The only other MP to declare their candidacy was Penny Mordaunt, who conceded to Mr Sunak shortly before the number of nominations for each candidate was due to be announced.
The 42-year-old will replace Liz Truss, who was forced to announce her resignation just 45 days into her premiership after her disastrous mini-budget spooked financial markets and caused division within her own party.
Shortly after winning the leadership, he is reported to have told Conservative MPs that the party needed to "unite or die".
Mr Sunak, the former chancellor of the exchequer, later said in a televised speech that the country faced "a profound economic challenge".
"We now need stability and unity," he said from the Conservative Party's headquarters in London.
"And I will make it my utmost priority to bring our party and our country together.
"Because that is the only way we will overcome the challenges we face and build a better, more prosperous future for our children and our grandchildren.
"I pledge that I will serve you with integrity and humility, and I will work day in, day out, to deliver for the British people."
Mr Sunak will meet with King Charles III at Buckingham Palace on Tuesday morning where he will be appointed prime minister and asked to form government.
Sunak last candidate left standing
Former prime minister Boris Johnson pulled out of the race on Sunday night, saying that the country and the Conservative Party needed unity.
Candidates had until 2pm on Monday (midnight AEDT) to gain the necessary 100 nominations from Conservative MPs, with Mr Sunak running out to an early lead.
Several minutes before the announcement of the final candidates was set to be made Ms Mordaunt released a statement indicating she was withdrawing, saying "Rishi has my full support".
"We all owe it to the country, to each other and to Rishi to unite and work together for the good of the nation," she said.
"There is much work to be done."
Ms Truss also congratulated her replacement, just seven weeks after Mr Sunak congratulated her for being in the same position.
"Congratulations @RishiSunak on being appointed as Leader of the Conservative Party and our next Prime Minister," she tweeted.
"You have my full support."
Mr Sunak will come into the role in a politically and financially turbulent time for the UK.
The Conservative Party is languishing in the polls with opposition party Labour enjoying a 32 point lead, and he will also have to rebuild the country's economy as it slides into recession.
Labour's deputy leader, Angela Rayner, said "nobody voted for this" and called for an election.
"The Tories have crowned Rishi Sunak without him saying a word about what he would do as PM," she wrote on Twitter.
"He has no mandate, no answers and no ideas.
"The public deserve their say on Britain's future through a general election."
In a speech to Conservative MPs shortly after winning the leadership, Mr Sunak ruled out holding an early general election.
General elections are held every five years in the UK, and the next one is not due until January 2025.
Britain's first Hindu prime minister
At 42, Mr Sunak will be the youngest person to become prime minister in the UK since Lord Liverpool in 1812.
He is also the first Hindu to hold office.
First elected as an MP in 2015, Mr Sunak capped off a meteoric rise from the backbench in February 2020 when Boris Johnson appointed him chancellor of the exchequer.
He was praised for a COVID-19 economic rescue package, including a costly jobs retention program that averted mass unemployment, but later faced criticism for not giving enough cost-of-living support to households.
The relationship of Mr Sunak and Mr Johnson would later break down – Mr Sunak's resignation from the role on July 5 was the catalyst for half of the Johnson cabinet quitting.
Born to Indian migrant parents in the port city of Southampton in England's south, Mr Sunak was educated at the exclusive Winchester College boarding school where he was school captain.
Mr Sunak is the 30th British prime minister to be educated at Oxford University, graduating with a philosophy, politics and economics degree in 2001, and also gained an MBA at Stanford University in the US in 2006.
After Oxford he worked for American investment bank Goldman Sachs before stints at several hedge funds where he accumulated a significant amount of wealth.
It was at Stanford that he met his future wife Akshata Murty, daughter of billionaire businessman NR Narayana Murthy, founder of IT company Infosys.
The couple have two daughters and an estimated combined wealth of 730 million pounds (A$1.3bn), making him one of the wealthiest prime ministers to ever hold office.
His wife's non-domiciled tax status caused controversy earlier this year when it was revealed she may have avoided up to 20 million pounds (A$35m) in UK tax by saying her permanent home was outside Britain.
Ms Murty later announced she would change her tax arrangements.
