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Prince William and his wife Kate are expecting their third child, Kensington Palace announced on Monday, adding that she would not be attending a planned engagement due to a severe form of morning sickness.

The news comes on the week that the royal couple's eldest child, four-year-old Prince George, begins school in London. They also have a daughter, two-year-old Princess Charlotte.

"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 palace said in a statement.

William is second in line to the throne and his grandmother Queen Elizabeth II, along with members of both families, was said to be "delighted" at the news.

The palace confirmed that "as with her previous two pregnancies, the Duchess is suffering from Hyperemesis Gravidarum", a severe form of morning sickness.

"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," it said.