Prince William and wife, Kate, the Duke and Duchess of Cambridge, have announced that they are expecting their third child.

Kensington Palace made the pregnancy announcement via their offical Twitter account after the Duchess of Cambridge was forced to cancel her attendance at a Monday engagement.

"The Duke and Duchess of Cambridge are very pleased to announce that The Duchess of Cambridge is expecting their third child."

As with her other two pregnancies, the duchess is suffering from hyperemesis gravidarum, or acute morning sickness. She is being cared for at the royal couples apartment in London's Kensington Palace.

The Duke and Duchess welcomed their first child Prince George in 2013 and thier second child, a girl in 2015. 

The sex of their unborn child was not revealed.

The baby will be fifth in line to the British throne, after grandfather Prince Charles, father William and elder siblings George, 4, and Charlotte, 2.

The popular Harry becomes sixth in line to the throne.