Mr and Mrs Johnson announced the birth of a “healthy baby girl” at a London hospital earlier on Thursday.
A spokeswoman for the couple said: “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.”
The couple announced they were expecting their second child together at the end of July.
The Press Association are reporting that Boris Johnson was present with Carrie throughout the birth.
The birth of a sister to the couple’s son Wilfred came the morning after the Prime Minister announced England is entering Plan B to combat the spread of the Omicron variant of coronavirus.
As well as facing anger over the latest restrictions in England, Mr Johnson is battling the fallout of an allegedly rule-breaking Number 10 Christmas party last year which forced the resignation of senior Government aide Allegra Stratton.
Also on Thursday morning, the Conservatives were fined £17,800 over the controversial donation to help cover the lavish refurbishments to the couple’s Downing Street flat.
The new arrival means Mr Johnson has at least seven children, although he has frequently obfuscated when asked exactly how many.
Labour leader Sir Keir Starmer tweeted: “Congratulations to Carrie and Boris Johnson on the birth of their daughter.
“Wishing your family health and happiness.”
Wilfred Lawrie Nicholas Johnson was born in April last year and was named in tribute to the doctor who helped saved the Prime Minister’s life when he was admitted to hospital with Covid-19 earlier that month.
The couple married during a secretly-planned wedding at Westminster Cathedral in May.
Mr Johnson, 57, has four children with his barrister ex-wife Marina Wheeler.
It emerged during a court hearing in 2013 that he had fathered a daughter during an affair when he was London mayor in 2009.
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