The Duchess of Cambridge has been admitted to a west London hospital after going into labour.
Catherine was admitted to the Lindo Wing at St Mary's Hospital where Prince George was born in July 2013.
Kensington Palace said earlier that "labour is progressing as normal" for Catherine, who was taken to the hospital by car.
The duchess travelled from the palace with her husband Prince William and was admitted at 06:00 BST (05:00 GMT).
Officials have said the duchess and the prince do not know the sex of their unborn child.
The new baby will be fourth in line to the throne, behind his or her grandfather Prince Charles , father Prince William and brother Prince George.
On Thursday, during an official engagement, Prince Charles said he was hoping the duchess gave birth to a girl.
Prime Minister David Cameron told BBC Breakfast it was "very exciting news".
He added: "With the rest of the nation I'll be wishing them well today, hoping for a healthy and happy outcome."
Labour leader Ed Miliband said: "I want to pass on my best wishes to the Duchess of Cambridge. My thoughts are with her and the Duke of Cambridge today. I wish them well."
Deputy Prime Minister Nick Clegg tweeted: "Best of luck to the Duchess of Cambridge today, as she gives birth to her second child. The thoughts of the whole country will be with her."
Two police officers now guard the familiar brown door to the private Lindo Wing at St Mary's Hospital in west London.
The media areas are filled with camera crews and reporters from around the world.
The small group of royal fans who've camped outside the hospital for two weeks at last know a baby is on the way.
The statement from Kensington Palace was emailed to royal correspondents at 06:34 BST confirming that the Duchess of Cambridge was in the early stages of labour.
It's a brief statement, just two sentences, and that will be it for now.
Kensington Palace has made it clear that there will be no running commentary on the duchess's labour.
The next news we will hear will be the announcement that a baby has been born.
Both Prince William and his brother Prince Harry were also born at the Lindo Wing.
Catherine, who is thought to be overdue, is being looked after by consultant obstetrician Guy Thorpe-Beeston.
He will be joined in the delivery room by Alan Farthing, the Queen's surgeon-gynaecologist.
Fans of the Royal Family have been gathering near the hospital with flags and banners in recent days.
But unlike at the time of Prince George's birth, the world's press were stopped from camping outside in advance.
The birth will be announced in an email to the press. Two minutes later, a tweet will be posted on the Kensington Palace Twitter feed - @KensingtonRoyal.
There will also be the traditional custom of placing a paper announcement outside Buckingham Palace.
Catherine went on maternity leave on 27 March when she carried out her last official public engagements before the birth.
On that occasion, she and Prince William met Baroness Lawrence at the Stephen Lawrence Centre and saw projects run by the charity eXceL Project in London.
Former press secretary to the Queen, Dickie Arbiter, said the duchess was carrying out royal duties until recently.
He said: "She wasn't letting anything stand in her way. She was feeling fit, if a little bit heavy."
He added: "She was supporting William, she was supporting the Royal Family and doing as much as she could."
He told BBC News the pregnancy was a "global phenomenon".
He added: "It is almost as big as it was two years ago. And as the day wears on I think it will get bigger."