The Duke and Duchess of Cambridge are expecting their third child, Kensington Palace has announced.
The Queen and both families are said to be “delighted with the news”.
As with her previous two pregnancies, the duchess, 35, is suffering from hyperemesis gravidarum, or severe morning sickness.
She will no longer carry out her planned engagement at the Hornsey Road Children’s Centre in London today.
Catherine is being cared for at Kensington Palace, the statement said.
The Duke and Duchess have one son, George, and one daughter, Charlotte, aged four and two.
With the previous two pregnancies, the couple announced them before the 12-week mark because of the duchess being unwell with hyperemesis gravidarum.
Hyperemesis gravidarum affects about one in every 200 pregnancies and results in severe nausea and vomiting – with one of the main dangers being dehydration.
The expectant child will become the fifth in line to the throne behind Prince Charles, Prince William, Prince George and Princess Charlotte.
If a boy, it will be the first time new rules to remove male primogeniture will be invoked.
The change – which stops royal sons taking precedence over their female siblings in the line of succession – came into force in March 2015.
The child will be the Queen’s sixth great-grandchild.
The last third-born monarch
To become King or Queen as the third-born royal child is rare – and has yet to happen within the current House of Windsor.
But the third child of George III and Queen Charlotte, William IV, took on the task and ruled from 1830 to 1837.
The Hanoverian king acceded to the throne aged 64 when his older brother, George IV, died without an heir.
He became the “heir apparent” when he was 62 and his other older brother, Frederick, Duke of York, died.