Prime Minister Boris Johnson and Carrie Symonds have announced that they are expecting a baby and that they have got engaged.
The couple's baby is due in the early summer.
A spokesperson 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."
They have been living together in 10 Downing Street since July and since adopted a dog, Dilyn, who was being cared for in a rescue home in the Rhondda before he found his new home.
Mr Johnson, 55, and Miss Symonds, 31, made history as the first unmarried couple to officially live together in Downing Street when they moved in last year.
Ms Symonds, a conservationist and former Conservative Party communications chief, first found herself making headlines when she was romantically linked to Mr Johnson earlier in 2019.
But her association with Mr Johnson dates back to when she worked on his successful re-election bid at City Hall in 2012.
Mr Johnson's union with Miss Symonds will be his third marriage.
Earlier this month, a court heard that the Prime Minister and his estranged second wife Marina Wheeler were preparing to end their marriage after reaching an agreement over money.
A family court judge approved a financial settlement and gave Ms Wheeler permission to apply for a divorce decree.
Mr Johnson is believed to be the first prime minister to be divorced in office in modern times.
Ms Wheeler and Mr Johnson, who have four children, separated in 2018 after marrying in 1993.
The last child born to a sitting prime minister belonged to David and Samantha Cameron, who welcomed Florence Rose Endellion into their family in August 2010.
