Prince William and his wife Catherine are expecting their third child, Kensington Palace announced this morning.
"The Duke and Duchess of Cambridge are very pleased to announce that The Duchess of Cambridge is expecting their third child," the palace said in a statement.
"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."
William is second in line to the British throne after his father Prince Charles.
The announcement came four days after commemorations to mark the tragic death of Princess Diana, William's mother, 20 years ago in Paris.
Prince William visited Malta in 2014 for the celebrations to mark the fiftieth anniversary of independence but his wife, who was pregnant with their second child, did not accompany him.
READ: Prince William thanks Malta, says Duchess hopes to visit
William and Catherine, or Kate as she is popularly known, have two children, George and Charlotte.
By having more than two offspring, William and Kate are following in the footsteps of the Queen and the Duke of Edinburgh, who went on to have four children - although there was a gap of 10 years between their second child, Anne, and third, Andrew.
Kate is one of three and had a happy home life with her sister, Pippa Matthews, and brother, James Middleton, and is close to both of her siblings.
But William - who is one of two siblings - may have needed more convincing.
On an overseas tour to Singapore in 2012, when he was asked by a group of teenagers how many children he would like to have, he said he was ''thinking about having two''.
The couple talked openly of having a family when they announced their engagement and planned ahead by saving the top of their wedding cake - traditionally served at the christening of a first-born.
On a royal tour of Poland in July 2017, Kate joked about having a third - after being given a present designed for newborns.
She speculated about the prospect of another child when she was offered a cuddly toy designed to soothe tiny babies during an event for start-up tech companies in Warsaw.
Saying thank you for the present, she turned to the Duke, laughing, and said: "We will just have to have more babies.''