Boris Johnson and Ms Symonds have said they are “very pleased” and revealed their first-born is due to arrive in the early summer. The couple are yet to announce a date for the wedding. A spokesman for the couple said: “The Prime Minister and Miss Symonds are very pleased to announce their engagement and that they are expecting a baby in the early summer.”
Ms Symonds, 31, is the first ever unmarried partner of a Prime Minister to live in 10 Downing Street.
The couple have been living inside Number 10 since Mr Johnson won the race to succeed Theresa May in July.
The shock announcement comes as the Prime Minister has been criticised for his lack of public appearances during the flood crisis in the UK.
A number 10 source said: “Boris and Carrie are both delighted at this news.
“They have both known for a while but have kept it under wraps until the pregnancy progressed.
“It partly explains why he has been lying low recently – although he works flat out and that won’t change.
“Carrie is thrilled and she will also continue to work on her environmental projects.”
READ MORE: Boris Johnson baby: ‘Carrie proposed to Boris in leap year engagement’
The couple were engaged in a relationship when the Prime Minister was still married to his second wife, Marina Wheeler.
Earlier this month, a court heard Mr Johnson and his estranged second wife Marina Wheeler were preparing to end their marriage after reaching a financial settlement.
Mr Johnson and Ms Wheeler have four children together and separated in 2018 after getting married in 1993.
Mr Johnson met his first wife, Allegra Mostyn-Owen, while they were students at Oxford University.
They married in 1987 but they decided to divorce six years later.
Former Scottish Conservatives leader Ruth Davidson was one of the first to congratulate the Prime Minister on the news.
Ms Davidson wrote on Twitter: ”Congratulations to Boris Johnson and Carrie Symonds.”
Former chancellor Sajid Javid, who resigned from Government after a dispute with Mr Johnson, offered the couple his congratulations, tweeting it was ”wonderful news!”
Communities Secretary Robert Jenrick said: “Congratulations Boris Johnson and Carrie Symonds – wonderful news!”
Rob Roberts, a newly-elected Welsh Conservative MP, said: “Congratulations to Boris Johnson and Carrie Symonds on their announcements today! Exciting times.”
Ms Symonds has already dipped her toe into public life when on August 16 last year she made her first public appearance since entering 10 Downing Street addressing what she called the “gigantic” climate crisis.
Ms Symonds is regularly referred to as a PR guru and indeed she has a very glittering background – her parents are Matthew Symonds, one of the founders of The Independent newspaper, and Josephine Mcaffee who was one of the paper’s lawyer.
She grew up in East Sheen, South West London, and was a pupil at the posh Godolphin and Latymer School in Hammersmith.
In her work as senior adviser at Oceana supporting the Bloomberg Foundation’s Vibrant Oceans Initiative she helps run the “Save the Oceans, Feed the World” Initiative and Oceana’s campaign to reduce the use of plastics.