Prince William and Kate are expecting their third child

ReutersBritain's Prince William, his wife Kate, and their children George and Charlotte pose in late October 2015.

Prince William and Kate are expecting their third child after George (4) and Charlotte (2), Kensington Palace announced today.

'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 Queen and members of both families are delighted with the news,' the palace said in its statement.

Kate is still in the first trimester of the pregnancy and, as has been the case with her first two pregnancies, is experiencing severe morning sickness, or hyperemesis gravidarum. Her engagements today have been cancelled.

The palace said: '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.' 

The new baby will be fifth in line to the throne, after Prince Charles, Prince William, George and Charlotte. This means Prince Harry will move to sixth in line.

Prince George starts school this week at Thomas's in Battersea, just south of the river near Chelsea in London.

Possible names are Alice for a girl and Philip for a boy.

Prince William was until recently a pilot for East Anglian air ambulance but left that job to become a full-time 'royal' after his grandfather, the Duke of Edinburgh, retired.