The Duchess of Cambridge has been admitted to St Mary's Hospital in London in the early stages of labour, Kensington Palace said today.
She travelled by car from Kensington Palace to the Lindo Wing of the hospital with the Duke of Cambridge.
The world's press have been camped outside St Mary's in Paddington for days in anticipation of the birth.
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The news that what had been dubbed by bored journalists 'the Great Kate Wait' was finally over was announced in a brief statement from Kensington Palace at 7.30am after rumours she had been spotted began circulating.
The statement read: "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."
[Royal baby: How will news of the birth be announced?]
The Duchess was taken to the Lindo wing just before 6am.
There was a strong police presence around the hospital and two police officers guarded the entrance to the private wing.
The Prince of Wales, who will become a grandfather for the first time when Kate's baby is born, was quizzed about the birth as he visited the National Railway Museum in York to mark the 75th anniversary of the Mallard locomotive.
But the heir to the throne revealed he was awaiting news like everybody else telling Sky News he knew "Absolutely nothing at the moment, we're waiting".
Kate and William, who spent the weekend at Kensington Palace, traveled without a police escort, their spokesman said.
He added: "Things are progressing as normal."
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.
Prices increase if the delivery is a difficult one or the mother has a caesarean section.
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But Kate is intending to have a natural birth and does not know whether she is going to have a boy or girl.
The Duke 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 in turn.
Well-wishers from around the globe began gathering outside Buckingham Palace today.
Tourists armed with cameras peered hopefully through the Palace gates on the off-chance of spotting the easel, due to be placed on the forecourt detailing confirmation of the birth.
Betting on the name of the royal baby, which will be third-in-line to the throne, has produced one favourite with a number of bookies - Alexandra.
Other monikers that have attracted royal fans include Charlotte, Diana, Elizabeth and Victoria, with George and James picked by those who think the new baby will be a boy.
William will take paternity leave from his job as an RAF search and rescue helicopter pilot.
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 crowned monarch.
It will be the 43rd sovereign since William the Conqueror if, as expected, it follows reigns by Charles then William.
The Duke is known to want a daughter while the Duchess is hoping for a son.
What will the royal baby look like?
When Kate met Guardsman Lee Wheeler, 29, during a a St Patrick's Day parade in Aldershot, Hampshire, she told him she did not know the sex of her baby.
The soldier said: "I asked her 'Do you know if it's a girl or boy?', and she said 'Not yet'.
"She said 'I'd like to have a boy and William would like a girl'. That's always the way."
Recent changes to the rules of succession mean if a girl is born she will not be leapfrogged by a younger brother at a later date.
The sex of an infant in direct line to the throne no longer determines whether he or she wears the crown.
Confirmation that the Duchess is in hospital also means the nation is aware that Kate is in labour.
This is something that would have been unthinkable when the last granddaughter-in-law of a reigning queen to give birth to a future monarch did so in the 1890s.
George V's wife Mary of Teck, who was then the Duchess of York, had the future Edward VIII in 1894 and the future George VI in 1895, but news of her 'confinement' was limited.
Dr Rowbotham, a social historian at Nottingham Trent University, said: "Pregnancy was not something that was publicly talked about then. It wasn't discussed. It was indelicate."
David Cameron told the BBC: "Best wishes to them, a very exciting occasion and the whole country is excited with them. So, everyone's hoping for the best."