The Prime Minister is already a father-of-five, however this will be the first child born to his 31-year-old partner.
Mr Johnson recently finalised his divorce with estranged wife Marina Wheeler, with whom he has two daughters and two sons.
While he was still married, he began dating PR guru and Tory adviser Ms Symonds, and the couple are believed to have been dating close to two years.
A spokesperson for the couple said tonight: ‘The Prime Minister and Ms Symonds are very pleased to announce their engagement and that they are expecting a baby in the early summer.’
The Prime Minister and Ms Symonds have also recently got engaged.