The Duke and Duchess of Cambridge are expecting their third child, Kensington Palace has announced.
The Queen is “delighted” with the news, the Palace said in a statement.
The Duchess is suffering from severe morning sickness, as she did with her previous pregnancies, forcing her to cancel a public engagement she had later today, the Palace added.
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.
“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.”
The latest addition to the Cambridge family will be a great-grandchild of the Queen and the Duke of Edinburgh.
He or she will be fifth in line to the throne, pushing uncle Prince Harry into sixth place.
But it is unlikely that, as a third born royal in direct succession, he or she will become king or queen.
The Prince of Wales is first in line, followed by William, Prince George and the couple’s daughter Princess Charlotte, who is two.
The news comes as the family prepares for a major milestone when Prince George starts his first day at school on Thursday.
The Duke and Duchess of Cambridge have chosen an independent school in Battersea, south London, where fees cost from £17,604 a year, for their four-year-old son.
George’s first day at school and the baby announcement comes at an important time for the family, which is now mainly based at their Kensington Palace apartment, rather than their Norfolk home, Anmer Hall.
William is now a full-time royal having quit his job as a pilot for the East Anglian Air Ambulance in July.
The Prince is due to attend the National Mental Health and Policing Conference in Oxford tomorrow.
Speculation will begin immediately on the sex and likely name of the couple’s third child, with Alice – popular with bookmakers last time – or Alexandra possibilities for a girl and Frederick, James or Philip for a boy.
It would be a surprise, given they chose traditional royal names for their first two children, if Kate and William now went for something more modern.
In July Kate hinted at a third child on the family’s royal tour of Poland.
Given a cuddly toy meant for a newborn, she said to William, laughing: ‘We will just have to have more babies’.
The Palace has not said when Kate, 35, is expected to give birth.
Number 10 tweeted Theresa May’s congratulations, posting: “PM – This is fantastic news. Many congratulations to the Duke and Duchess of Cambridge.”