Boris Johnson and his girlfriend Carrie Symonds are engaged and are expecting a baby in early summer.
The Prime Minister is already a father-of-five, however this will be the first child born to his 31-year-old partner.
A No.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.”