Kate was taken to the private Lindo wing of St Mary’s Hospital in Paddington, London, as she prepares to give birth to her second child.
The baby prince or princess will be fourth in line to the throne, the Queen’s fifth great-grandchild and a spare to the heir - a younger brother or sister for Prince George.
Kensington Palace said: “Her Royal Highness The Duchess of Cambridge was admitted at 06.00hrs to St. Mary’s Hospital, Paddington, London and is 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 palace added “labour is progressing as normal” for Kate, who was taken to the hospital from her home, which is a short drive to the hospital.
The Duchess, who was overdue, is being looked after by consultant obstetrician Guy Thorpe-Beeston, who is the surgeon-gynaecologist to the household. He will be joined in the delivery room by Alan Farthing, the Queen’s surgeon-gynaecologist.
William and Kate were keen to avoid the media circus surrounding George’s arrival in July 2013. Unlike last time, the world’s press have been prevented from camping outside the Lindo Wing for days in anticipation of the birth in what was dubbed the “Great Kate Wait”.
Press pens will now open outside the hospital as journalists, photographers and camera crews prepare to bide their time until confirmation of the baby’s arrival.
The birth will be declared in both conventional and contemporary ways.
After an announcement is emailed to the press, two minutes later there will be a celebratory tweet on the Kensington Palace Twitter feed - @KensingtonRoyal.
George became the first future British monarch to have news of his birth tweeted by a royal household, with @ClarenceHouse declaring: ‘’Her Royal Highness The Duchess of Cambridge was safely delivered of a son at 4.24pm.”
There will also be the traditional custom of placing a paper proclamation behind the iron railings of Buckingham Palace.
There will be less theatre around the transportation of the paper notice. Last time the couple’s press secretary was filmed handing over the foolscap-sized sheet, signed by the Duchess’s doctors, in a smart red leather folder to a car to be taken to the Palace. The aim this time is for the delivery to be made more discreetly.
Just like with her first-born, Kate is hoping for a natural birth and does not know whether she is having a boy or a girl.
With George, the Duchess gave birth 10-and-a-half hours after being admitted to hospital, but second-born babies often make an appearance earlier than the first.
Kate went on maternity leave on March 27 when she carried out her last official public engagements before the birth as she and William met Baroness Lawrence at the Stephen Lawrence Centre and saw projects run by the charity eXceL Project in London
The Duke and Duchess have thanked people for their ‘’warm wishes’’ ahead of the birth.
‘’They know that people are excited that Prince George will soon have a little brother or sister and it means a great deal to them that so many will be celebrating this important moment for their family,’’ a source said.
Bookmakers will be waiting to see whether they have to pay out on the name, delivery date, hair colour and weight of the baby.
Favourite names thought to be in the running include Alice, Elizabeth, Charlotte, Victoria or Diana for a girl and James, Richard, Arthur, Albert, Charles and Philip for a boy.
In 2013, William and Kate returned briefly to Kensington Palace with their son, allowing the Queen to pay them a visit and meet her great-grandson before the new family headed to the Middleton mansion near Bucklebury in Berkshire for several weeks to adjust to their new life.
This time, they will stay for a few days at Kensington Palace before decamping to their new country bolthole, Anmer Hall on the Sandringham estate in Norfolk.
William has been on unpaid leave after completing the first phase of his air ambulance helicopter training and is not due back at work until June 1.
While Kate is in labour, George is likely to be cared for by his full-time, live-in nanny Maria Teresa Turrion Borrallo.
The Duchess’s pregnancy was announced by Kensington Palace on September 8 last year when she was thought to be just six weeks pregnant. The news was revealed early because, just like when pregnant with George, Kate was suffering from extreme pregnancy sickness hyperemesis gravidarum.
She pulled out of a number of engagements and an overseas solo tour to Malta, but was back in the spotlight by the end of October, confessing at the Singapore state visit: ‘’I’ve been looking forward to getting out of the house, that’s for sure.’’
William has confessed he cannot wait for the baby to be born, describing George’s arrival as a life-changer, but adding: ‘’Number two is a game-changer.”
Kate revealed the baby was kicking and moving all the time during official engagements, and that she was struggling to get back up after bending down. She also said the baby was due mid to late April and she did not know whether it was a boy or a girl.
If the baby is a girl, she will be the first girl born to the British royal family to take the title princess for 25 years. She would also be the highest-ranking female in line to the throne.
A girl has not been born this high up the line of succession for nearly 65 years. Princess Anne was born third in line in 1950 and will be 12th in line following the new arrival. Princess Beatrice - currently the highest ranking woman in the line of succession - was born fifth in line in 1988.
It would also be the first time a great-granddaughter of a still-serving sovereign has been born in direct succession on the male line since 1897, when George VI’s sister Princess Mary was born.
There has not been a Princess of Cambridge born for 182 years. The last was King George III’s granddaughter Princess Mary Adelaide of Cambridge who was born in Hanover, Germany, in 1833.
With the length of a second labour lasting an average of five hours, the baby will be expected to arrive before the end of the day.
Ardent supporters of the couple have been camped outside the hospital for more than a week.
Kate was several days overdue. She was spotted twice this week leaving Buckingham Palace having reportedly gone for a swim, possibly to help bring on labour.
Parking restrictions outside the Lindo were extended for a further five days earlier in the week by Westminster City Council to Tuesday May 5 but the baby looks set to make a May Bank holiday appearance.
Prince Harry has missed the chance to meet his new niece or nephew having already returned for Australia. He will not get to see the baby until at least mid-May.