Prince William and his wife Kate, the Duchess of Cambridge, are expecting their third child, Kensington Palace has announced.
The announcement said the Queen and members of both families were "delighted" by the news.
Officials said the Duchess of Cambridge was suffering from Hyperemesis Gravidarum, a form of severe morning sickness, and would not carry out her engagements on Monday (local time).
The Duchess suffered from the sickness in her previous pregnancies and she is being cared for at Kensington Palace, the statement said.
William and Kate already have two children, Prince George and Princess Charlotte, who are aged four and two respectively.
The announcement came as a surprise, as there had been little indication that William's 35-year-old wife was pregnant.
Their third child would be fifth in line to the British throne.
William, 35, is a grandson of the Queen and the eldest son of Prince Charles, who is the first in line to the throne.
