Princess Kate has been admitted to St Mary's Hospital in London in the early stages of labour, according to Kensington Palace.
The palace said in a statement: "Her Royal Highness The Duchess of Cambridge has been admitted this morning to St Mary's Hospital, Paddington, London, in the early stages of labour.
"The Duchess travelled by car from Kensington Palace to the Lindo Wing at St Mary's Hospital with The Duke of Cambridge."
Duke of Cambridge William and his younger brother Prince Harry were born in the Lindo wing and the Prince and Princess of Wales famously posed on the building's steps in 1982 holding baby William.
But it is not known how long the Duchess will take off from her royal duties to care for her first child.
The new royal baby will be the Queen's third great-grandchild and is destined to be king or queen.
William wants a daughter while Kate is hoping for a son.
Kate 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 fiance of murdered TV presenter Jill Dando and the Queen's current gynaecologist.
The hospital's Lindo wing is a private obstetric unit, with prices starting at just under £5,000 for a normal delivery package over 24 hours, with consultants' fees around £6,000 extra depending on the care required.
Kate is intending to have a natural birth and she says she doens't know whether she's having a boy or a girl.