British Prime Minister Boris Johnson and his girlfriend Carrie Symonds yesterday announced their engagement and also said they were expecting their first child.
The couple said their baby would be born in the early summer and disclosed they were engaged at the end of last year.
A spokesman for the couple made an official announcement yesterday, saying: "The Prime Minister and Ms Symonds are very pleased to announce their engagement and that they are expecting a baby in the early summer."
Carrie Symonds (31) then posted a message on Instagram admitting she had kept the news of her engagement secret until now, saying: "I wouldn't normally post this kind of thing on here but I wanted my friends to find out from me.
"Many of you already know, but for my friends that still don't, we got engaged at the end of last year... and we've got a baby hatching early summer. Feel incredibly blessed."
The couple enjoyed a Caribbean holiday in Mustique at the end of December.
The announcement of their forthcoming wedding means Johnson will become the first prime minister to marry in office for nearly 200 years.
It's Johnson's third marriage. He was first married to Allegra Mostyn-Owen, an Oxford contemporary. His second wife, Marina Wheeler, a barrister with whom he has four children, reached a divorce settlement this February after 25 years of marriage.
Carrie Symonds became the first unmarried partner of a prime minister to live in Downing Street when she moved into No 10 in July last year, on Johnson becoming Tory party leader.
She is expected to continue to work on her environmental projects. It was revealed last week that Symonds had hired Sarah Vaughan-Brown, the former ITN communications chief, as her personal adviser. It is understood that she will be helping Symonds with her charity interests.
Government ministers, MPs and friends congratulated the couple on social media.
The new arrival will be the third baby born to a serving prime minister in recent history.
Tony Blair's wife Cherie gave birth to son Leo in May 2000, while David Cameron and his wife Samantha welcomed Florence in 2010.
Charlotte, James and even Winston are said to be the bookies' favourites for potential baby names. The couple already share a dog, Dilyn, who the PM said helped keep his stress down during last December's election campaign.
Symonds came to public prominence when it emerged that she was in a relationship with Johnson, who at the time was still married but separated from Wheeler, with whom he has two daughters and two sons.
The couple have chosen to live at the larger four-bedroom flat at No 11 Downing Street instead of the smaller two-bedroom official residence at No 10.
Symonds has maintained a relatively low profile since moving into No 10, although she was photographed among the PM's staff welcoming him to Downing Street last July - leading to the pink and red floral midi dress she was wearing selling out within hours.
In June last year police were called in the early hours to the home in Camberwell, south London, which Symonds was at the time sharing with Johnson, after a neighbour heard screaming during an apparent row.