Latest: Prince William's wife Kate on Saturday gave birth to a girl, who will be fourth in line to the British throne, Kensington Palace said in a statement.
"Her Royal Highness The Duchess of Cambridge was safely delivered of a daughter at 8.34am," the statement said.
The Duchess travelled by car from Kensington Palace to the Lindo Wing at St Mary's Hospital with The Duke of Cambridge (Prince William)," it said.
British Prime Minister David Cameron sent his best wishes in a tweet.
"The whole country will wish her well," he wrote.
The child -- a prince or princess -- will be fourth in line to the throne after Queen Elizabeth II's eldest son Prince Charles, his eldest son William, and big brother George who was born in 2013.
From baby fashion to specially-minted coins, here is an A to Z on the birth of Prince William and his wife Kate's second child, due to be born shortly after his mother went into labour earlier on Saturday.
A is for ANNOUNCEMENT. A proclamation will be displayed on an easel at the gates of Buckingham Palace, while news of the birth will be tweeted by Kensington Palace and emailed to the press. Gun salutes will also mark the new arrival.
B is for BROTHER. The new royal baby will be 21 months younger than older brother Prince George, who is directly in line to become king one day.
C is for CHRISTENING. The baby will be baptised wearing a replica of a lace and satin gown that has been used since 1841. Tradition dictates the Archbishop of Canterbury Justin Welby, England's highest cleric, will conduct the christening.
D is for DUTY. Once an adult, a lifetime of royal duty on behalf of the sovereign awaits.
E is for EDUCATION. The royal youngster is set to enjoy exclusive schooling. Both William and his brother Prince Harry went to the elite boarding school Eton.
F is for FASHION. The new royal will be a trendsetter before it can walk and talk -- anything George has been seen wearing has flown off the shelves.
G is for GRANDPARENTS. William's father Prince Charles, the heir to the throne, and his wife Camilla are expected to visit the baby in hospital shortly after the birth, along with Kate's parents Michael and Carole Middleton.
H is for HOME. The new baby will grow up at the Cambridges' apartment in London's Kensington Palace and Anmer Hall, a country house on Queen Elizabeth II's private Sandringham estate in eastern England.
I is for INTEREST. The birth will spark huge interest around the world, with international media camped outside the hospital doors waiting for the first glimpse of the new royal.
J is for JORDAN. The baby will be baptised with water from the River Jordan.
K is for KATE. The 33-year-old duchess suffered acute morning sickness again during her second pregnancy, causing her to miss royal engagements.
L is for LINE OF SUCCESSION. The baby will be born fourth in line to inherit the throne, behind George and directly in front of uncle Harry.
M for MEMORABILIA. The Centre for Retail Research reckons the royal baby will bring retail sales of around £60 million to £70 million ($88-103 million, 84-98 million euros), rather than the £247 million boost attributed to George.
N is for NAMES. Royal babies often get several Christian names, with George getting three. Bookmakers' favourites for a girl are Alice, Elizabeth, Charlotte, Alexandra and Victoria; for a boy, James, Arthur, Henry, Philip and Albert.
O is for OBSTETRICIANS. The medical team will be led by Guy Thorpe-Beeston, the surgeon-gynaecologist to the royal household, who will be assisted by his predecessor Alan Farthing.
P for PRIMOGENITURE. Laws which took effect in March ending male primogeniture mean girls born after October 28, 2011 can no longer be overtaken by younger brothers in the line of succession.
Q is for QUEEN ELIZABETH II. The baby will be the monarch's fifth great-grand-child and her 17th descendant.
R is for ROYAL MINT. They will produce a £5 coin in celebration and gift a special "lucky" silver penny to 2,015 babies born on the same day.
S is for ST MARY'S HOSPITAL. The baby will be born in the private Lindo Wing of the hospital in Paddington, central London. A two-room suite costs £6,750 a night, though consultants' fees can double that.
T is for TITLE. The baby will be known as His/Her Royal Highness Prince/Princess (name) of Cambridge.
U is for UNCLE HARRY. William's fun-loving brother is coming out of the army and so may have more time to babysit. The baby will also have Auntie Pippa and Uncle James on its mother's side.
V is for VICTORIA. The baby will be the great-great-great-great-great-grandchild of 19th-century queen Victoria.
W is for WILLIAM. The 32-year-old duke is training to become an air ambulance pilot. He will take two weeks' paternity leave. He could face a two-hour drive to London when Kate goes into labour.
X is for XX or XY? As with George, the couple do not know the sex of the baby.
Y is for YELLOW. Bookmakers are taking bets on the colour of Kate's dress when she leaves hospital, with a spring-like yellow among the favourites for the first baby pictures that will zip round the world.
Z is for ZZZ... William and Kate can expect months of sleepless nights, but their nanny will be on hand to ease the burden.
Kate in labour
The BBC has reported that Prince William's pregnant wife Kate, Duchess of Cambridge, is now in labour.
She is at the private Lindo wing at St Mary's Hospital in London.
Swirling rumours created a brief media frenzy outside St Mary's Hospital earlier, fed by photos on social media of police activity and a sniffer dog.
"We're getting close," said Kathy Martin, 50, one of around a dozen of royalist faithful decked out in the colours of the Union Jack flag who have slept outside the London hospital for a week in tents and on benches.
Martin was delighted at the box of croissants and apple turnovers from Kensington Palace -- William and Kate's official residence in the British capital.
"They must have seen in the news that we are camping here for a week now in the cold, and it is really cold, I can tell you that," she said.
"It shows how nice those people are. The box was wrapped in pink ribbons. It could be a sign they are having a girl," she told AFP.
Fellow fan Maria Scott, 44, added: "We get a lot of abuse.
"Some say we're idiots or freaks and that the monarchy does nothing for us but we don't believe that," she said.
A royal source said: "The Duke and Duchess had seen them on the news reports and wanted to let them know they were thinking of them by sending some breakfast".
The royal family has not said whether the baby, who will be fourth in line to the throne, will be a boy or a girl and has not revealed the due date.
The Daily Telegraph has reported that the baby was due last week and that the birth would probably have to be induced some time this week.
One fan, John Loughrey, said he was sure the baby would be born on Wednesday because it was William and Kate's fourth wedding anniversary, but fellow camper Terry Hutt predicted it would be on Thursday.