After winning the Conservative leadership contest, the former foreign secretary has met the Queen and been officially asked to form a government.
The former foreign secretary beat her rival Rishi Sunak to win the Conservative leadership race on Monday and will replace Boris Johnson in Number 10.
In a break with tradition, Ms Truss travelled to Balmoral, where the Queen is on holiday, for her formal appointment.
It was thought to be best for the Queen, 96, not to travel back to London due to her intermittent mobility issues.
Ms Truss is the Queen’s 15th prime minister. The first was Winston Churchill.
Now that the so-called kissing the ring ceremony has passed, Ms Truss will return to Downing Street to begin assembling her cabinet.
Several well-known figures have resigned ahead of the expected reshuffle, including Priti Patel, the home secretary, and Nadine Dorries, the culture secretary.
Other MPs will be eagerly waiting by their phones for a job offer, or their marching orders.
Earlier, Mr Johnson gave his last speech as prime minister to a packed Downing Street before travelling to Balmoral to formally tender his resignation to the Queen.
He said he would “get behind Liz Truss every step of the way” and told the Conservative Party that “it is time to get behind” his successor.
Mr Johnson also hinted at a possible political comeback, comparing himself to Cincinnatus, a Roman statesman who resigned and returned to his farm before, according to tradition, becoming leader again in later years.
“Like Cincinnatus, I am returning to my plough and I will be offering this government nothing but the most fervent support,” Mr Johnson said.
After Ms Truss returns from Scotland, she will make her own speech on the steps of Number 10 to set the agenda for her leadership.
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