The Queen, 94, sent a private message of good wishes to the Prime Minister and his fiancee, Buckingham Palace said. Ms Symonds, 32, gave birth to a baby boy in a London hospital on Wednesday morning.
The birth was announced shortly before 10am 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 Prime Minister and Ms Symonds would like to thank the fantastic NHS maternity team.”
It is understood the Prime Minister was present throughout the delivery.
Mr Johnson returned to work in Number 10 following the birth but missed Prime Minister’s Questions.
The family are planning to live in their Downing Street flat along with their rescue dog Dilyn.
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.
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The baby’s name is not yet known.
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