The Duchess of Cambridge has gone into labour.
She travelled by car from Kensington Palace to St Mary's Hospital in Paddington, west London, the BBC reports.
The Duchess, who is in the early stages of labour, was accompanied by her husband.
She is to give birth in the Lindo Wing at St Mary's, where both Prince William and Harry were born.
The couple do not know the gender of their first child.
There is likely to be no more news until the official announcement of the birth.
As the world eagerly awaits the arrival of the future King or Queen, we round up 10 royal baby facts you may have missed: The royal baby is third in line to the throne behind Charles and William
In October 2011 the Government announced the 16 Commonwealth countries where the Queen is head of state had accepted changes to the rights of succession, enabling a first-born daughter of the Duke and Duchess of Cambridge to take precedence over younger brothers.
The baby pushes Prince Harry, currently third in the line of succession, to fourth.
However, being third in line the baby is unlikely to become King or Queen for another 50 or 60 years. You'll catch a glimpse of the royal baby soon
The Duke and Duchess are expected to pose for photos with their newborn as they leave hospital, just as Prince Charles and Princess Diana did with William.
"While it is a deeply personal and private event, the Duke and Duchess of Cambridge realise this is a time of national celebration and that there will be vast interest in the baby," a source told the national media.
"They realise many people will want to share in their happiness." Doughnuts, mobile phone covers and potties are among the royal baby souvenirs being produced to celebrate the birth
Krispy Kreme is selling limited edition doughnuts with blue or pink centres, while FisherPrice has launched a range of Prince and Princess themed potties that play royal tunes. These will sit alongside more traditional items such as tea-towels, mugs and plates.
Meanwhile, babies born on the same day as the Duke and Duchess of Cambridge's first child will receive a free "lucky" silver penny, the Royal Mint has announced.
The penny, which will be presented in a pink or blue pouch, has been marked with the year 2013 to commemorate the year of birth, and features a shield of the Royal Arms.
For more information, visit www.royalmint.com. The royal baby is likely to live in a newly-refurbished apartment in Kensington Palace
But apartment 1A is no studio – it's a four-story, 21-roomed mansion, whose former occupant was Princess Margaret. The refurbishment reportedly cost £1 million. The royal baby will have the attention of mum and dad
Unlike royal parents of generations past, William and Kate will not turn to a full-time nanny to help raise their baby.
In their quest for normality, the couple have hinted at a hands-on approach. And the royal baby is expected to spend a great deal of time away from the royal household, with grandparents Michael and Carole Middleton.
Prince William is also rumoured to be considering giving up his job as a search and rescue helicopter pilot for the Royal Air Force in Wales, in order to spend more time with his family.
Royal child-raising has transformed over just a few generations. Princess Diana was criticised in the 1980s for taking young William with her on a royal tour to Australia, and broke with years of tradition by openly showering her sons with affection. The birth of the royal baby will be announced at the gates of Buckingham Palace
To retain the theatre of the occasion, a proclamation signed by the doctors who delivered the baby will be displayed on an easel at the gates of Buckingham Palace.
For the first time, the birth announcement was also released via social media.
It will also be displayed in lights on the BT Tower. The royal baby will be the second ever to have dad present in the delivery room
Proving himself to be a modern dad, Prince William will be at Kate's side when she gives birth in the Lindo Wing at St Mary's Hospital.
A sign of changing times, when Charles was born his father Prince Philip was playing squash.
Prince Charles was the first royal father to be present at the birth of his sons.
And Prince William was the first heir to the British throne to be born in a hospital. It was then a radical choice not to give birth in a royal residence. Royal baby fever is set to boost the economy by around £243 million this summer
According to data from the Centre for Retail Research, at least £80 million will be spent on souvenirs and £25 million on festivities celebrating the birth.
Professor Joshua Bamfield, director the CRR, said: "It is no great surprise that this type of feel good event provides retailers with a welcome boost, with consumer hype generating additional spending.
"These are difficult times for retailers and every catalyst for consumer spending should be utilised, as this will bring the economic recovery one step closer." The royal baby has already had a song written in its honour
Composer Paule Mealor, who wrote a piece of music for the royal wedding, has written a lullaby for the Cambridge's child called 'Sleep On'. It will be sung by classical superstar Hayley Westenra. The royal baby might not have a surname
A surname will not necessarily be required, as the baby will have the title HRH Prince or Princess and will be referred to in this way.
However, if Kate and William want to include a surname they can choose between Mountbatten-Windsor, Wales or Cambridge.
