Boris Johnson's fiancee Carrie Symonds has given birth to a baby boy, the couple have announced.
A spokeswoman for the Prime Minister and his partner said both mother and baby are "doing very well" after the birth in a London hospital on Wednesday morning.
It is understood Mr Johnson was present throughout the birth.
The spokeswoman 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."
So thrilled for Boris and Carrie. Wonderful to have a moment of unalloyed joy!
— Matt Hancock (@MattHancock) April 29, 2020
Mr Johnson, 55, and Ms Symonds, 32, announced in March that they were expecting a baby in "early summer", and that they had become engaged at the end of last year.
The news of the birth comes just weeks after the PM was discharged from intensive care where he had been battling coronavirus.
Ms Symonds also suffered symptoms of the disease.
She returned to Downing Street after joining Mr Johnson as he convalesced at Chequers, but it is not yet known which hospital she gave birth in.
Mr Johnson has previously suggested he intends to take paternity leave, although it is not known if this remains the case given the coronavirus crisis.
He will not appear at Prime Minister's Questions in the Commons on Wednesday afternoon.
Labour leader Sir Keir Starmer said the birth was "wonderful news" as he congratulated the couple on Twitter.
Acting Liberal Democrat leader Sir Ed Davey said: "Many congratulations to the PM and Carrie Symonds on the birth of their son!"
Chancellor Rishi Sunak said it was "great to hear Downing Street is getting a new resident".
And the PM's father, Stanley Johnson, told the PA news agency he was "absolutely delighted" and "thrilled" by the birth of his grandson.
Mr Johnson and Ms Symonds made history as the first unmarried couple to officially live together in Downing Street when they moved in last year.
The new arrival is the third baby born to a serving prime minister in recent history.
Great to hear Downing Street is getting a new resident.
— Rishi Sunak #StayHomeSaveLives (@RishiSunak) April 29, 2020
Tony Blair's wife Cherie gave birth to son Leo in May 2000, three years after her husband's first election victory, and David Cameron and wife Samantha welcomed daughter Florence in 2010.
The last babies born to prime ministers before Leo and Florence arrived more than 150 years ago.
The baby is Ms Symonds' first child, while Mr Johnson is known to have fathered five.