His fiancee, Carrie Symonds, gave birth to their son on Wednesday (28 Apr) morning at an NHS hospital in London, with both mother and baby said to be doing “very well”.
How many children does Boris Johnson have?
Mr Johnson is the father of six children - three girls and three boys.
Four of his children are with his ex-wife Marina Wheeler, and his third daughter was conceived during an affair with arts consultant, Helen Macintyre.
His newborn son with 32 year old Symonds is his sixth child.
The couple are yet to give their new son a name, but he was born before 10am on 28 April 2020.
A spokeswoman for the couple said, "The Prime Minister and Ms Symonds are thrilled to announce the birth of a healthy baby boy at a London hospital earlier this morning. Both mother and baby are doing very well.
"The PM and Ms Symonds would like to thank the fantastic NHS maternity team."
Who are Boris Johnson’s older children?
Lara Lettice is the eldest of Mr Johnson’s children, at the age of 27, and was conceived ahead of her parents’ wedding.
The Prime Minister married barrister Marina Wheeler in 1993. They later divorced, after 25 years of marriage. The couple reached a final settlement in February 2020.
Lara is a writer, editor and broadcaster, and has achieved an impressive career in journalism, having penned articles for The Spectator, British Vogue and the Evening Standard.
She is now the features editor at online style magazine SHOWStudio, and launched her own publication, (un)fold magazine, in 2016.
Milo Arthur is the second eldest child at age 25, and was educated at Westminster School.
He was said to excel at sports and was a keen cricket player, coming running up in the Funaki prize, a long-running award for artistic endeavour.
He later went on to study at London's School of Oriental and African Studies, after which he completed a five-month internship at men's magazine, Esquire Middle East, based in Dubai.
Cassie Peaches is the third of Mr Johnson’s children and is aged 23. She studied at Highgate School in North London and is a writer.
Theodore Apollo is the youngest of the Prime Minister’s children with his ex-wife, and is 21 years old. He attended Cambridge University, but it is not known what subject he is studying.
Stephanie is the fifth child of the brood and was born in 2009.
Her mother is art consultant Helen Macintyre, with whom Mr Johnson had an extramarital affair. It is not believed Stephanie’s older half-siblings, or her father, have any contact with her.
The third baby born to a serving PM
Mr Johnson and Ms Symonds announced in February that they were expecting a baby in "early summer" and that they had become engaged at the end of last year.
They made history as the first unmarried couple to officially live together in Downing Street when they moved in last summer, after Mr Johnson won the Conservative Party leadership contest.
The new arrival is the third baby born to a serving Prime Minister in recent history.