The Duchess of Cambridge is in labour, as the world's media gather outside the hospital awaiting news of the latest addition to the Royal Family.
Catherine and the Duke of Cambridge arrived by car at St Mary's Hospital in Paddington, west London, at 06:00 BST.
Kensington Palace said she was in the early stages of labour, which was progressing as normal.
The couple do not know the sex of their first child, who will be third in line to the throne.
The world's media have been camped outside St Mary's for days in anticipation of the birth. The due date had never been officially announced but had widely been expected to be mid-July.
Royal vehicles were seen at a back entrance to the hospital at about 06:00 BST, with the announcement coming from Kensington Palace 90 minutes later.
The couple, both 31, travelled to the hospital without a police escort, their spokesman said.
The duchess is being tended by a top medical team led by the Queen's gynaecologist Marcus Setchell, who delivered the Countess of Wessex's two children. Alan Farthing, gynaecologist to the Royal Household, will be assisting.
She will give birth in the hospital's private Lindo Wing, where Prince William and his brother Prince Harry were born.
There is a strong police presence around the hospital and two police officers are guarding the entrance to the private wing. The media scrum parked outside is having to contend with the heat on what is the hottest day in the UK since July 2006.
BBC royal correspondent Peter Hunt said the duchess was thought to be one week past her due date.
"The royal statement contains 45 words. All there will be to sustain the world's media until the baby is born," he tweeted.
Last week, the Queen joked about the imminent arrival of the newest member of her family, saying she hoped the baby would be born before she went on holiday to her private Balmoral estate in Scotland.
During a visit to the National Railway Museum in York, the Prince of Wales was asked by the members of the press and the public about the birth of his first grandchild but he did not divulge any information.
Prince William has been with his wife on annual leave and will have two weeks' paternity leave from his job as an RAF search and rescue helicopter pilot.
It is believed the next time the couple will be seen in public is when they appear on the steps of the hospital with their baby.
Plans for how the birth will be announced were amended by Kensington Palace officials on Monday evening.
A formal press release containing details of the birth will now be issued shortly before a formal signed bulletin leaves the hospital for Buckingham Palace.
It is custom for news of royal births and deaths to be attached to the railings of the palace, but in this case it will be displayed on an ornate easel in the forecourt of the palace.
The Queen, senior members of the Royal Family and the duchess's family - if they are not at the hospital - will be told about the birth first.
Then a royal aide will take a bulletin, signed by key medical staff, from the hospital to the palace under police escort.
Under new laws to succession the baby will remain third in line to the throne regardless of gender and the gender of any subsequent siblings.
Prime Minister David Cameron spoke about the excitement surrounding the imminent arrival, and also about the recent changes to the rules of succession.
Speaking on BBC Radio 4's Woman's Hour, he said it was "pretty much a done deal".
'Thoughts and prayers'
All the nations of the Commonwealth have agreed, he said, even if the changes had not yet passed through their parliaments.
The Archbishop of Canterbury, who is likely to carry out any christening, has sent his best wishes to the couple.
"My thoughts and prayers are with Kate and the whole family on this enormously special day," tweeted the Most Reverend Justin Welby.
There is global interest in the royal birth and it has been the subject of much speculation, particularly when it comes to names.
Bookmaker William Hill says George is the favourite for a boy and, after a flurry of bets, Alexandra - one of the Queen's middle names - for a girl. Charlotte is also thought to be a contender.
The new royal baby will be the Queen's third great-grandchild. It will be expected to take the throne following reigns by Prince Charles and then Prince William.
The couple announced they were expecting their first child in December after the duchess was admitted to hospital suffering from severe morning sickness.
Since the announcement, the duchess has carried out 19 days of public engagements before going on maternity leave in the middle of June.
Catherine's final public appearance before the birth was at the Trooping the Colour ceremony in June.
The Duke and Duchess of Cambridge were married at Westminster Abbey in April 2011, watched on television by hundreds of millions of people around the world.