The Duchess of Cambridge was admitted to hospital at 0600BST this morning in the early stages of labour.
She was accompanied by her husband, Prince William, the Duke of Cambridge.
This was about the same time as she arrived at the exclusive Lindo Wing in Paddington, West London, 21 months ago when royal watchers waited for the arrival of the next in line to the throne.
The new baby will follow his or her brother in the succession regardless if it is a boy or girl because of the changes made in the law regarding primogeniture.
That means if the baby is a girl – as is widely anticipated by book makers – she will be fourth in line to the throne behind her brother Prince George even if the couple have another son.
Kensington Palace tweeted the news this morning.
The Duchess was driven to the private hospital with her husband.
A spokesman from Kensington Palace said Prince Harry was among the first to be told his nephew or neice is on the way, with The Queen, the Prince of Wales and other members of the Royal Family “informed as soon as it was practically possible”.
Meanwhile the kittens at Battesea Dogs and Cats home have bucked the national trend and decided they think the new baby will be a boy.
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But not everyone is excited, with Graham Smith, chief executive of campaign group Republic saying “it’s not big news”.
Despite the huge interest and people camping out for weeks to catch a glimpse of the couple and their newest addition to the family, he said “it’s not news most people are interested in beyond brief curiosity.
“To spend more than 30 seconds talking about the non-story or a royal baby is a complete over-reaction.”