The new was confirmed in a press release sent out by Kensington Palace on Monday morning.
"The Queen and members of both families are delighted with the news," the statement continued.
Princess Catherine is, as with her first two pregnancies, suffering from a condition known as Hyperemesis Gravidarum – an acute version of morning sickness. She cancelled an engagement originally planned for the day of the announcement.
The Duke and Duchess had their first child, Prince George, on 22 July 2013. The four-year-old prince has just started school.
The couple's second child, Princess Charlotte, was born on 2 May 2015.
Messages of congratulation quickly began flying in across social media:
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