On Wednesday 29 April, a spokesperson for the couple stated: "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."
Mr Johnson and Ms Symonds announced they were engaged and expecting a baby together in February.
The announcement means that 55-year-old Mr Johnson will become the first UK prime minister in 250 years to marry while in office.
In a post shared on her private Instagram account, Ms Symonds wrote: “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. Feel incredibly blessed.”
Mr Johnson and 31-year-old Ms Symonds have been romantically involved since 2018.
In July last year, Mr Johnson addressed the country during his first televised speech as prime minister, having assumed the position earlier that month.
Ms Symonds attended the event in support of her partner, being dubbed “first girlfriend“.
Within hours of her appearance, the £120 pink and red floral midi Ghost dress she had been wearing sold out from the fashion label’s website.
Here is everything you need to know about Ms Symonds:
What is her background and her job?
Ms Symonds, the daughter of The Independent co-founder Matthew Symonds and lawyer Josephine Mcaffee, was born on 17 March 1988 and raised in southwest London.
Her paternal grandfather was newspaper editor John Beavan, Baron Ardwick, a Labour Party MEP.
After studying theatre studies and art history at the University of Warwick, she began her career as a press officer for the Conservative Party.
While working at the Conservative Campaign Headquarters, she campaigned for Mr Johnson during the 2010 London Conservative Party mayoral selection, which resulted in the future prime minister being selected to stand in the 2012 mayoral election.
Her career also saw her work for former culture secretary John Whittingdale, in addition to becoming a media special advisor for Sajid Javid when he was in the Department for Communities and Local Government.
In 2018 at the age of 29, Ms Symonds became the youngest-ever head of communications of the Conservative Party.
She left the position in 2018 in order to take up a public relations role at Oceana, an ocean conservation charity.
On the Oceana website, it states that Ms Symonds is “passionate about protecting the oceans and marine life, particularly by reducing plastic pollution”.
On her Twitter profile, where she describes herself as a “conservationist”, she frequently shares information about plastic pollution and the environment.
Ms Symonds has also used her platform to speak out about issues such as ending female genital mutilation (FGM).
How did the couple meet?
While it is unclear when the couple met, their relationship was confirmed shortly after Mr Johnson announced he would be divorcing his wife of 25 years, Marina Wheeler, in September 2018.
Mr Johnson and Ms Wheeler have four children together, ranging in age from 20 to 26.
The prime minister’s fifth child was born in 2009 following an extra-marital affair.
Mr Johnson’s child with Ms Symonds will be the sixth that he has publicly acknowledged.
It is believed the politician may have fathered another child outside of marriage.
In July 2019, Ms Symonds moved to Downing Street with Mr Johnson, becoming the first unmarried partner of a prime minister to move into the residence.