The Duchess of Cambridge has been admitted to hospital in the early stages of labour and is "progressing as normal", Kensington Palace has said.
Kate Middleton travelled by car from Kensington Palace to the Lindo Wing of the hospital with the Duke of Cambridge.
She 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.
The world's press have been camped outside St Mary's in Paddington for days in anticipation of the birth.
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.
But Kate is intending to have a natural birth and does not know whether she is going to have a boy or girl.
The news that 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."
The Duchess was taken to the Lindo wing just before 6am.
Kate and William, who spent the weekend at Kensington Palace, travelled without a police escort, their spokesman said.
He added: "Things are progressing as normal."
There was a strong police presence around the hospital and two police officers guarded the entrance to the private wing.
Carly Gargett, 31, an event manager from Sydney, Australia, who lives in London, said: "I have been doing live Facebook updates to all my friends back in Australia. They were texting me this morning saying 'What's going on?' because I'd missed it happening so early.
Miss Gargett said she thinks the Duchess would love to have a girl, adding: "Every girl would like a girl princess," she said. I'm picking Charlotte as the name - I have a feeling in my waters. I don't think I'll be doing a lot of work today, I have the royal baby cam live feed to my phone, I am so excited."
Dickie Arbiter, a former press secretary to the Queen, said the birth will be a historic first. "There will be three heirs in waiting while the sovereign is fit and well, and that's a first," he said.
"The Queen will be the first to be informed because William will telephone her as soon as something happens. The Middletons might come down but the royals won't because it demands another level of security and the last thing they'll want is to disrupt the hospital. It doesn't really warrant a visit," he added. "Diana was in for 22 hours; she had the baby one day and left the next. Kate might do the same thing."
Well-wishers beginning to congregate outside the Palace
We'll post updates as soon as they are available
To celebrate the royal birth, we'll be taking free baby announcements for two weeks in the Oxford Mail. Details are here