The prime minister and his wife Carrie Johnson have announced the birth of their second child, a baby girl.
In a statement, a spokeswoman for the couple said: "The prime minister and Mrs Johnson are delighted to announce the birth of a healthy baby girl at a London hospital earlier today.
"Both mother and daughter are doing very well. The couple would like to thank the brilliant NHS maternity team for all their care and support."
They have not revealed if the little girl has a name yet.
Mr Johnson is understood to have been present throughout the birth and Downing Street said the PM will spend some time with his family but did not say if he would take any paternity leave.
"As ever he has to balance that with his responsibilities as Prime Minister in leading the country, which he will continue to do.