Symonds, 32, had said previously that their baby was due to “hatch” in the early summer. It was not clear if Johnson, 55, would take paternity leave given the government is facing the worst health crisis since the 1918 influenza outbreak.
“The Prime Minister and Ms Symonds are thrilled to announce the birth of a healthy baby boy at a London hospital earlier this morning,” a spokeswoman said. “Both mother and baby are doing very well.
“The PM and Ms Symonds would like to thank the fantastic NHS maternity team.”
There was no announcement of a name.
The new arrival tops a tumultuous month for Johnson; he returned to work on Monday after recuperating from COVID-19, which had left him gravely ill in intensive care at the peak of the CCP virus outbreak.
Symonds, a former public relations executive, also had symptoms of the CCP (Chinese Communist Party) virus, also known as the novel coronavirus, but recovered more swiftly.
Baby in Downing Street
Few British leaders have had babies while in office, although it has become more common among recent occupants of Downing Street.
David Cameron’s wife, Samantha, gave birth to their daughter, Florence, in 2010 and Tony Blair’s wife Cherie gave birth to their son, Leo, in 2006.
Before that, Frances, the wife of John Russell, a 19th-Century Whig and Liberal politician, gave birth to two children in 1848 and 1849 while he was in office.
“Great to hear Downing Street is getting a new resident,” said Johnson’s finance minister, Rishi Sunak.
Opposition Labour Party leader Keir Starmer said: “Wonderful news. Many congratulations to Boris Johnson and Carrie Symonds.”
Symonds and Johnson have been living together in Downing Street since he became prime minister in July. They announced in February that they were expecting their first child and that they were engaged to be married.
Johnson, once dubbed “Bonking Boris” by Britain’s riotous tabloid media, has a complicated private life.
He was once sacked from the Conservative Party’s policy team while in opposition for lying about an extra-marital affair. He has been divorced twice and refuses to say how many children he has fathered.
In 2013, when Johnson was mayor of London, appeal court judges ruled that the public had the right to know he had an extra-marital affair with a woman who gave birth to his daughter.
Johnson’s previous marriage was to Marina Wheeler, a lawyer. They had four children together but announced in September 2018 that they had separated. They divorced earlier this year.
Symonds announced her engagement to Johnson in February.
“Many of you already know, but for my friends that still don’t, we got engaged at the end of last year… and we’ve got a baby hatching early summer,” she said.
“Feel incredibly blessed.”
By Guy Faulconbridge and Elizabeth Piper