The former Kate Middleton, 31, travelled by car to the hospital, and is expected to give birth in the private Lindon Wing of the hospital, where her husband and his younger brother, Prince Harry, were born to Diana, Princess of Wales, and Prince Charles.
She was admitted without fanfare or a police escort at about 6 a.m. local time, according to palace officials, who informed the media about 90 minutes later.
Her medical team is led by royal gynecologist Dr. Marcus Setchell.
The baby will be third in line to the British throne, and should eventually succeed grandfather Charles and father William as king or queen of Britain and 15 other countries, including Canada and Australia.
Birth announcement to come in stages
But little else is known about the eagerly awaited royal baby, from how it will arrive, to its gender or name. It is not known, for instance, if the Duchess of Cambridge will have a natural birth or deliver by a caesarean section.
Royal watchers must wait to be told of the baby's arrival from the palace, which plans to reveal the news through a mixture of tradition and social media.
Palace officials have said the first hint will come when a royal aide emerges from the hospital with a signed bulletin carrying the Buckingham Palace letterhead. The bulletin will be given to an official who will be driven to the palace, where it will be posted on an easel in public view in front of the building.
At the same time the bulletin is posted, there will be an official announcement on Twitter and the media will be formally notified. The document will give the baby's gender, weight and time of birth.
It could be some time before the baby's name is made public — it took a week for William's name to be announced, while Charles's name remained a mystery for a month.
But it is the baby's gender that is of particular interest because the prospect of Kate's pregnancy prompted a change to laws of succession to ensure a daughter would not be passed over for the crown by a younger brother. Boy or girl, the child will be third in line to the throne and the prospective future monarch.
The birth of a new heir to the throne has been breathlessly anticipated since William and Kate wed on April 29, 2011, in a lavish ceremony at Westminster Abbey.
When news of a royal pregnancy was announced, there was rejoicing in many households in Britain and throughout the world.
Despite a rough start to her pregnancy, when she was hospitalized for acute morning sickness, the Duchess of Cambridge made a number of public appearances that were halted only near the end of her term.
Since the duchess has cut back on her royal duties, media outlets have been clamouring for position outside of the hospital in anticipation of the birth, jockeying to secure the best vantage point for filming William and Kate emerging, babe in arms.
Settling into the role of parents
Officials have said William plans to take two weeks of paternity leave and then return to his military duties as a search-and-rescue helicopter pilot in Wales.
His tour of duty is scheduled to wrap up around September, and he and Kate are expected to move from their isolated cottage on the island of Anglesey off the coast of Wales to Kensington Palace in central London.
But major refurbishment works at the palace likely won't be finished until at least a month or two after the royal birth — meaning William and Kate will most likely have to make do with their current temporary home in London, a two-bedroom property at the palace.
Come autumn, however, the family will be able to move into their permanent London home, Apartment 1a at Kensington Palace — a four-storey house with a nursery, 20 rooms and a private garden.