The second-in-line to the throne is expected to be at his wife’s side as she gives birth to their third child in six years, joining Prince George, four, and Princess Charlotte, six.
No specific due date was given when Princess Catherine, still known informally as Kate Middleton, announced last September that she was expecting her third child, though the announcement did say the baby was due in April.
The couple have not disclosed whether they are expected a boy or a girl, although bookmakers are giving a slight edge in the betting market to Prince George and Princess Charlotte welcoming a brother.
She has been taken to a private wing of St Mary’s Hospital in London this morning after going into labour, a short statement from Kensington Palace said.
The Duchess of Cambridge has, with all three pregnancies, suffered from hyperemesis gravidarum, a severe form of pregnancy-related morning sickness.
Mary and Arthur are currently the joint-favourites with oddsmakers for the new baby, who will become fifth in line to the throne.
Royal names such as Victoria and Albert are also among the favourites, with the names of Prince William’s parents, Charles and Diana, distinct outsiders.