Prime Minister Boris Johnson and his fiancee Carrie Symonds have announced the birth of a son.
A spokeswoman for the PM and his partner said both mother and baby are "doing very well".
It is understood Mr Johnson, who has just recovered from coronavirus, was present throughout the birth, at an NHS hospital in London.
But he has now returned to work in Downing Street, No 10 said, where he is leading the response to the pandemic.
He is expected to take a "short period" of paternity leave at some point later this year, Downing Street said.
The couple have received messages of congratulation from across the political spectrum, and Mr Johnson's father Stanley said he was "absolutely delighted" and "thrilled" by the birth of his grandson.
Downing Street declined to say whether the baby was born prematurely, and did not provide details of the weight, timing, nature or location of the birth.
"The PM and Ms Symonds would like to thank the fantastic NHS maternity team," Downing Street said.
The Queen has sent a private message of good wishes to the couple to congratulate them on the birth of their son, Buckingham Palace said.
The PM's weekly audience with the the Queen is due to take place later by telephone.
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.
They are the first unmarried couple to move into Downing Street together.
The baby is Ms Symonds' first child, while Mr Johnson is known to have fathered five.
The family are planning to continue living in the flat above Number 11 Downing Street and it's understood their dog, Dilyn, will also be remaining in residence.
Mr Johnson returned to work on Monday, after a battle with coronavirus which saw him spend three nights in intensive care. Ms Symonds also suffered symptoms of the disease.
Ms Symonds said on social media that she had spent a "worrying" week in bed with the symptoms of the virus while the PM was self-isolating with the the disease.
She later sent Mr Johnson baby scans and daily messages while he was in hospital, to keep his morale up.
Australian prime minister Scott Morrison and Japanese prime minister Shinzo Abe were among the first international leaders to offer their congratulations.
Mr Johnson's Conservative colleagues have also been congratulating the couple on social media, with Health Secretary Matt Hancock saying: "So thrilled for Boris and Carrie. Wonderful to have a moment of unalloyed joy!"
Labour leader Sir Keir Starmer also offered his congratulations on the "wonderful news".
Whatever their political differences, he said at Prime Minister's Questions, "as human beings I think we all recognise the anxiety that the prime minister and Carrie must have gone through in these past few weeks - unimaginable anxiety.
"I really hope that this brings them incredible relief and joy."
Sir Keir's spokesman said the Labour leader had held "constructive talks" about the coronavirus crisis with the prime minister, by telephone, on Wednesday afternoon.
There was speculation that Mr Johnson would take part in his first Prime Minister's Questions since recovering from coronavirus on Wednesday.
But his place was taken Foreign Secretary Dominic Raab, who has been deputising for him.
Mr Raab said: "I'm sure the whole House will want to join with me in sending congratulations and our very best wishes to them."
He also paid tribute to the key workers who have lost their lives fighting coronavirus and wished a happy 100th birthday to Captain Tom Moore who has raised over £29m for the NHS.
The four children from Mr Johnson's second marriage, to barrister Marina Wheeler, are in their 20s. He was reported to have reached a divorce settlement with Ms Wheeler in February.
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.
Mr Cameron tweeted his "heartfelt congratulations" to Mr Johnson and Ms Symonds, adding: "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!"
Scottish First Minister Nicola Sturgeon tweeted: "Some good news - sending congratulations to Carrie and the PM. And wishing health and happiness to the wee one."