He became the third Prime Minister in two months, in a country plagued by unprecedented instability and a deep social crisis.
After his victory in the British Conservative Party, Rishi Sunak met this Tuesday morning King Charles III, who officially appointed him head of government.
It is the first time that Charles III appoints a head of government.
The current Downing Street tenant, Liz Truss, was received by Elizabeth II on September 6 during an audience at the Scottish castle of Balmoral.
The 96-year-old sovereign died two days later.
Read alsoUnited Kingdom: who is Rishi Sunak, who will be appointed Prime Minister this Tuesday by King Charles?
"Trust is earned and I will earn yours," assured the new Prime Minister in front of the steps of 10 Downing Street, also promising that his government will "demonstrate integrity".
Rishi Sunak also pledges to fix the "mistakes" made under Liz Truss and warns of "difficult decisions ahead".
Liz Truss, who resigned after just 44 days in office on Thursday, made her last speech outside 10 Downing Street around 10.15am, wishing her successor 'success', then went to Buckingham Palace to hand in her resignation to the King.
Rishi Sunak was then received in audience by the king.
“A profound economic challenge”
Former banker and Minister of Finance, Rishi Sunak becomes, at 42, the youngest head of government in the contemporary history of the United Kingdom after a meteoric rise in politics.
He is also the first of Indian origin and the first from a former British colony.
After his victory within his formation on Monday, he promised "stability and unity".
"Bringing the party and the country together will be my top priority," he said in a brief address.
"The UK is a great country, but there is no doubt that we face a profound economic challenge."
Rishi Sunak becomes the head of government of a country facing a serious economic and social crisis.
Inflation exceeds 10%, the highest in the G7.
Energy prices are soaring, as are food prices.
The risk of recession hovers.
He will also have to calm the markets, shaken by the Truss government's budget announcements at the end of September, all of which have since been canceled in disaster.
Rishi Sunak comes to power in a period of unprecedented instability: he is the fifth British Prime Minister since 2016, when the country chose in a referendum to leave the European Union.
He is the third head of government in two months.