Kate Middleton, 31, has been admitted to hospital and is in the early stages of labor.
The Duchess of Cambridge was taken to St Mary’s Hospital in London.
The long wait was over but there was likely to be no more news until the official announcement of the birth.
The baby will be Prince William and Kate Middleton’s first child and will be third in line to the throne.
Prince William is with Kate Middleton at the private Lindo Wing of St Mary’s Hospital in Paddington, west London, where he himself was born in 1982.
Palace officials chose to make the announcement that Kate Middleton has gone into labour public in an attempt to balance her “dignity” with the fact that social media makes it almost impossible to keep her baby’s imminent arrival a secret.
“The Duchess of Cambridge has been admitted this morning to St. Mary’s Hospital, Paddington, London in the early stages of labour,” a royal spokesman said.
“The Duchess travelled by car from Kensington Palace to the Lindo Wing with The Duke of Cambridge.”
The world’s press have been camped outside St Mary’s for days in anticipation of the birth. The due date had never been officially announced but had widely been expected to be mid-July.
Prince William has been with his wife on annual leave and will have two weeks’ paternity leave.
Kate Middleton’s final public appearance before the birth was at the Trooping the Colour ceremony in June.
As a result of recent, long-anticipated changes in the law, the baby will also be made an HRH (His or Her Royal Highness) and given the title Prince or Princess of Cambridge.
If she is a girl she will, one day, become Queen, just as a boy will become King.
He or she is also destined to become a future head of the armed forces, supreme governor of the Church of England and head of the Commonwealth, which covers 54 nations across the world, and subsequently head of state of 16 countries.
Royal vehicles were seen at a back entrance to the hospital at about 06:00 BST.
It is believed the next time Kate Middleton and Prince William will be seen in public is when they appear on the steps of the hospital with their baby.
Kate Middleton is being tended by a top medical team led by the Queen’s former gynaecologist Marcus Setchell, who delivered the Countess of Wessex’s two children.
Assisting him is Alan Farthing, the former fiancé of murdered TV presenter Jill Dando and the Queen’s current gynaecologist.
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