British Prime Minister Boris Johnson and his partner, Carrie Symonds, have announced they are expecting a baby and are engaged to be married, Downing Street confirmed on Saturday.

No further information is being provided by the Prime Minister's office at this stage.

The last time a child was born to a sitting prime minister was in 2010, when David Cameron and his wife, Samantha Cameron, welcomed their daughter Florence.

Johnson, 55, has four children from his second marriage.

Symonds, 31, has enjoyed a successful political career, working as part of the campaign team that helped to get Johnson re-elected as mayor of London in 2012.

Symonds went on to become the youngest-ever director of communications at the party, at 29, and analysts say she was crucial in tidying up Johnson's famously disheveled persona during the leadership campaign.

Johnson comfortably won the Conservative Party leadership election last July and was appointed prime minister.

Symonds joined his aides outside the door to Number 10 Downing street as he made his first speech as leader.