The Queen has sent a private message congratulating the prime minister and his fiancée on the arrival of their baby son.
The monarch sent the message just hours before she and Boris Johnson will have their weekly audience.
The audiences were not held while the prime minister was in hospital, but the pair resumed the weekly talks when he was recovering at Chequers.
Although he missed Prime Minister’s Questions, the call, with the Queen in Windsor Castle, is planned for Wednesday evening.
Johnson will postpone his paternity leave until later in the year as the country battles the coronavirus pandemic.
Carrie Symonds announced she and the prime minister were expecting a baby in February and she said she was due in “early summer”.
Number 10 announced the birth of their son on Wednesday morning, and said both mother and son were “doing very well”.
“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 spokeswoman said.
“The PM and Ms Symonds would like to thank the fantastic NHS maternity team.”
It is understood Johnson was present throughout the birth.
The weight and other details of the baby have not yet been released.
Symonds, 32, accompanied Johnson, 55, to the Commonwealth Day Service at the beginning of March, one of the Queen’s most important events of the year.
She was understood to then be self-isolating, and at the beginning of April confirmed she’d had some symptoms of coronavirus but was recovering.
The Queen also sent a message to the prime minister when he was admitted to hospital – the same evening as her royal message was broadcast on television.
Johnson returned to work at Number 10 on Monday.
Johnson is the 14th prime minister to hold office during the Queen’s reign.