Britain's Prince William and his wife Kate Middleton are expecting their third child, Kensington Palace has announced.
In a statement the palace said the duchess was suffering from severe morning sickness, as she did with her previous pregnancies, so a public engagement she had later today had been cancelled.
The statement said: "Their royal highnesses the Duke and Duchess of Cambridge are very pleased to announce that the Duchess of Cambridge is expecting their third child.
Read the press release in full ↓ pic.twitter.com/vDTgGD2aGF
— The Prince and Princess of Wales (@KensingtonRoyal) September 4, 2017
"The Queen and members of both families are delighted with the news.
"As with her previous two pregnancies, the Duchess is suffering from Hyperemesis Gravidarum.
"Her royal highness will no longer carry out her planned engagement at the Hornsey Road Children's Centre in London today.
"The Duchess is being cared for at Kensington Palace."
Just weeks ago, the Duchess joked about having a third child while on a tour of Poland and Germany with William, George and Charlotte.
After being given a present designed for newborns while in Warsaw, she laughed and turned to William, saying: ''We will just have to have more babies.''
Kensington Palace said in a statement: "Their Royal Highnesses the Duke and Duchess of Cambridge are very pleased to announce that the Duchess of Cambridge is expecting their third child.
"The Queen and members of both families are delighted with the news."
Kate had been due to visit the Hornsey Road Children's Centre in London to hear about the mental health impact of becoming a mother.
Clarence House said of heir to the throne Charles and the Duchess of Cornwall: "They are delighted at the news."
The couple's third child will be born fifth in line to the throne, bumping uncle Prince Harry out of the top five for the first time and into sixth place.
The Prince of Wales is first in line, followed by William, George and two-year-old Charlotte.
"Fantastic, great! Very, very happy for them" Prince Harry responds to the news The Duchess of Cambridge is expecting another Royal baby pic.twitter.com/j42ShzqLjb
— Sky News (@SkyNews) September 4, 2017
Asked for British Prime Minister Theresa May's response to the announcement, a Downing Street spokeswoman said: "It is fantastic news and she passes on her congratulations to the Duke and Duchess of Cambridge."
The announcement comes as William and Kate are preparing to send their eldest child off to school for the first time.
Four-year-old future king George starts at fee-paying independent school Thomas's Battersea in south London on Thursday.
The past year has already been one of big changes for the Cambridges.
Their main home is now their Kensington Palace apartment rather than their Norfolk mansion Anmer Hall and William quit his job as a pilot for the East Anglian Air Ambulance in July to become a full-time royal.