In his first public address, Sunak vowed to serve with "integrity and humility", saying the UK faces a profound "economic challenge".
ADVERTISEMENTFormer finance minister Rishi Sunak is set to become the UK's next prime minister, after winning the Conservative leadership election on Monday.
His last remaining challenger, Penny Mordaunt, bowed out of the race, having failed to get the backing of enough Conservative MPs.
Speaking on Thursday afternoon, Sunak said he would serve with "integrity and humility", adding the UK faces a "profound economic challenge".
"We now need stability and unity," he said in a brief statement. "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."
He also paid tribute to out-going leader Lis Truss, who resigned last week.
Sunak will become prime minister in the coming days.
Opposition leaders in the UK are calling for a general election.
There had been a 15:00 CET deadline for candidates in the contest to replace Liz Truss to submit the names of a hundred Conservative politicians who were backing them, as per the contest's rules.
In a short statement, Tory party official Sir Graham Brady said only one person had received enough support, making Sunak the winner.
Announcing that she was pulling out of the race, Mordaunt wrote on Twitter: "We all owe it to the country, to each other and to Rishi to unite and work together for the good of the nation."
Sunak was the former Chancellor of the Exchequer in Boris Johnson's government, but quit in July, helping trigger a rebellion that bought Johnson down.
Johnson himself had hoped he could challenge Sunak and replace Truss, though pulled out of the leadership contest on Monday.
Sunak will be Britain's third prime minister in just seven weeks as he succeeds Truss.
A 42-year-old multi-millionaire and former hedge fund manager, he will be the UK's first PM from a British-Asian background, and the youngest leader in two centuries.
Former PM Truss congratulated Sunak, writing on Twitter that he had her "full support", while Thersa May another recent UK leader said he would provide "the calm, competent, pragmatic leadership our country needs."
What might some of Rishi Sunak's policies be?
Sunak comes to the helm at a challenging time in UK politics, with the country gripped by a cost of living crisis, political turmoil and the Ukraine war still grinding on.
He already set out many of his policy positions when he was in the race to succeed Boris Johnson, and although some of these might change, his previous comments can give some indication of the direction his new administration might take:
In August, Sunak said the country faced a “profound economic crisis”.
ADVERTISEMENTAs finance minister between February 2020 and July 2022, he set Britain on course to have its biggest tax burden since the 1950s. He increased corporation tax from 19% to 25%.
He also set out higher public spending but simultaneously promised more discipline and to cut waste.
During the summer leadership campaign, he criticised Truss’s tax-cutting agenda, saying he would instead only cut taxes once inflation had been brought under control. At the time he outlined a plan to cut income tax from 20% to 16% by 2029.
Sunak has backed the independence of the Bank of England and stressed the importance of government policy working alongside the central bank to tame inflation, not exacerbating it.
Political challenges
One of Sunak’s first challenges will be to show he can control a Conservative Party that has a large majority in parliament but is riven with factions that differ on key issues like Brexit and immigration as well as economic management.
ADVERTISEMENTHigher taxes will be strongly opposed by some in the party; others will oppose spending cuts in key areas like health and defence.
Winning the leadership contest, and becoming prime minister, is only the first step in uniting a party that has ousted its last two leaders over internal differences, lost another after a short-term policy implosion, and spent years arguing with itself over how to leave the European Union.
Sunak supported Brexit in the 2016 referendum but is still seen by some on the right of the party as too sympathetic to the EU.
The key issue of trade with Northern Ireland is still being negotiated with Brussels. Sunak would face pressure to get a deal that rewrites parts of the initial exit agreement without conceding to a lasting EU say over trade between Britain and Northern Ireland.
He will also face calls to follow through on government promises to control immigration into the country, an issue which many Conservative lawmakers see as critical to winning over voters at the next election.
ADVERTISEMENTOn Northern Ireland, Sunak previously said he would push on with legislation designed to unilaterally overrule the Brexit deal while still trying to negotiate with the EU. The bill, currently in parliament, has been heavily criticised by the EU.
On Brexit more broadly, in August he promised to “keep Brexit safe” and set up a new governmental unit to review EU regulations that still apply in British law.
Previously, Sunak has said he was proud to come from a family of immigrants, but he believed Britain must control its borders, and would retain a plan to deport asylum seekers to Rwanda.
He also refused to rule out Britain’s withdrawal from the European Court of Human Rights, plus has said the overall number of asylum claims should be capped, and the rules about who is eligible tightened.