Boris Johnson and his fiancee Carrie Symonds have been congratulated by political figures on the birth of their son.
Ms Symonds, 32, was delivered of a boy at an NHS hospital in London on Wednesday morning and both mother and baby are said to be “doing very well”.
Mr Johnson returned to work in Number 10 following the birth, but missed PMQs in the Commons. He spoke to Labour leader Sir Keir Starmer later on Wednesday afternoon.
Downing Street said the Prime Minister, who is currently grappling with the coronavirus crisis, would take a “short period” of paternity leave later in the year.
The new family are planning to live in their Downing Street flat along with their dog Dilyn, the Number 10 spokesman confirmed.
Downing Street declined to say whether the baby was born prematurely and the spokesman was unable to provide details of the weight, timing, nature or location of the birth.
The baby’s name is not yet known.
The birth was announced shortly before 10am on Wednesday by a spokeswoman for the couple.
She 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.”
The news 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 his country residence, Chequers, in Buckinghamshire.
The PM’s father, Stanley Johnson, told the PA news agency he was “absolutely delighted” and “thrilled” by the birth of his grandson.
Labour leader Sir Keir said it 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 Conservative former prime minister David Cameron sent his “heartfelt congratulations”, and said: “Sam and I are thrilled for you both! Sorry we didn’t leave the cot – but the climbing frame should still be in the garden!”
Foreign leaders, including Australia’s PM Scott Morrison and Japanese premier Shinzo Abe, also sent their wishes.
The Archbishop of Canterbury, Justin Welby, tweeted: “Wishing them every blessing and happiness.”
The couple 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.
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 others.