Boris Johnson will officially take up residence in Downing Street, and Number 10 confirmed his girlfriend Carrie Symonds will be living with him.
A Downing Street spokeswoman said: "The PM is officially moving in today and, yes, his partner will be living there."
Despite speculation that Mr Johnson may have needed new furniture to be paid for by the state, the spokeswoman said "there's no cost to the taxpayer".
Ms Symonds, a former Tory spin doctor, will not have any publicly-funded staff working for her, Number 10 indicated.
The spokeswoman said: "In relation to her living at Number 10 there won't be any additional cost to the taxpayer."
They will be the first unmarried couple to officially live in Number 10.
The couple had been living together at Ms Symonds's flat in Camberwell, south London, until a well-publicised row recorded by neighbours last month.
As Theresa May left Downing Street last week, removal vans were seen taking away her belongings.
Both Mrs May and former chancellor Philip Hammond left their flats in Downing Street as the handover to Mr Johnson's new administration took place.
Meanwhile Hollywood actor Dwayne 'The Rock' Johnson jokingly claimed Boris Johnson is his cousin, but later backtracked on the statement on Twitter.
When the new Prime Minister was mentioned to the Jumanji star on Good Morning Britain, he replied: "He's family, by the way; you don't know that, but now all of Britain knows... he's my cousin.
"It's like looking in the mirror, every time I see Boris."
Johnson followed up his comments with a tweet he would later delete in which he commended a comment made last week by the Tory politician in the first speech of his premiership - that "the people are our bosses".
The former wrestler tweeted: "Breaking: PM Boris Johnson is in fact my cousin (though we clearly look more like twins).
"Jokes aside, PM did say something in his speech I liked - 'the people are our bosses'.
"100% agree. The people/audience/consumer will always matter most. #ourboss"
After the tweet received some disparaging comments, it was removed and Johnson appeared to backtrack. "Well according to the people, maybe we're not related after all," he tweeted.