William and Kate, the Duke and Duchess of Cambridge, already have two children -- Prince George and Princess Charlotte -- aged four and two.
"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.
Kate is suffering from Hyperemesis Gravidarum, a form of severe morning sickness, and will not carry out her engagements today, it said.
The Queen and both families are said to be "delighted with the news".
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