The Duke and Duchess of Cambridge are expecting their second baby, it was announced today.
The couple said they were “very pleased” to reveal Prince George will have a new brother or sister.
The Queen and members of both William and Kate’s families are “delighted” at the news, Kensington Palace said.
As with her first pregnancy, the Duchess is suffering from hyperemesis gravidarum - acute morning sickness.
She is being treated by doctors at Kensington Palace and royal officials said she will no longer be attending a planned engagement in Oxford today.
Kate, 32, was due to join William in formally opening Oxford University’s £21 million centre dedicated to the study of China. William will still attend, as planned, Kensington Palace said.
The full statement announcing the pregnancy said: “Their Royal Highnesses the Duke and Duchess of Cambridge are very pleased to announce that the Duchess of Cambridge is expecting their second child.
“The Queen and members of both families are delighted with the news.
“As with her first pregnancy, the Duchess is suffering from Hyperemesis Gravidarum.
“Her Royal Highness will no longer accompany the Duke of Cambridge on their planned engagement in Oxford today. The Duchess of Cambridge is being treated by doctors at Kensington Palace.”
It comes around six weeks after Prince George celebrated his first birthday.
The couple’s second child will become fourth in line to the throne, shifting Prince Harry further down the line of succession.
The Duchess of Cambridge’s announcement will throw into doubt whether she will be able to attend her first solo overseas tour, set to take place in Malta later this month.
Kate’s first pregnancy was revealed when she was just a few weeks pregnant after she was admitted to hospital suffering from severe morning sickness in December 2012.
Hyperemesis gravidarum, which affects 3.5 per 1,000 pregnancies, causes severe vomiting and can lead to dehydration, weight loss and a build-up of toxins in the blood or urine, called ketosis.
It is understood that Kate’s pregnancy has not passed the 12-week stage - a similar position to when she was struck by the illness when in the early stages of her first pregnancy.
A Kensington Palace spokesman said the Duchess’s attendance at future events would be decided on a “case by case” basis.
Kate is scheduled to be among the guests at the opening ceremony of the Invictus Games in London on Wednesday evening, along with William, Harry and the Prince of Wales and Duchess of Cornwall.
The following day she is due to attend a solemn Drumhead service at Lee Valley Athletics Centre with the Duke and Harry, and later with William to watch Invictus competitors taking part in matches and heats.
Kate is due to make her first official solo overseas tour in two weeks’ time, from September 20-21.
A spokesman said: “We will be planning this on a case by case basis - it’s too early to talk about Malta and Invictus.”
Rumours about a second child have been rife since earlier this year.
William fuelled the speculation in April when he told a woman who presented him with a lace shawl: “You might have to make another one soon.”
The rumours were quashed later when Kate drank wine at a vineyard and took a white-knuckle jet boat ride.
Then in July one bookmaker suspended betting on an announcement
Today’s news sparked betting on aspects from baby names to the child’s hair colour in what bookmakers described as “the biggest novelty market of the year”.
It generated excitement around the globe, with the hashtag “royalbaby” trending worldwide and in the UK.
Former England rugby star Mike Tindall, husband of William’s cousin Zara Phillips, said he found out about the new baby when it broke on the news.
He told talkSport: “I just sent them a message saying it was nice to hear the news on the radio.”