- Duchess of Cambridge has given birth to her third child at St Mary's Hospital in Paddington, London
- Kate, 36, gave birth after being taken by car before 6am this morning to hospital's private Lindo Wing
- Supporters have decked out their tents in Union Jacks and put up flags and posters around the site
- Kate gave birth to George in 10-and-a-half hours after admission, and Charlotte in two-and-a-half
The Duke of Cambridge today brought his children Prince George and Princess Charlotte to meet their new baby brother nearly six hours after Kate gave birth to a boy weighing 8lbs 7oz on St George's Day.
Prince William unbuckled George from his car outside the Lindo Wing of St Mary's Hospital in Paddington, London, before he got out in his school uniform, then took Charlotte out of the car and she shyly waved to the cameras.
He then led them to meet their new brother, who is the Queen and the Duke of Edinburgh's sixth great-grandchild. Bookmakers have reported a surge in bets on the name Arthur for the royal baby, who is fifth in line to the throne.
Earlier, Prince William appeared outside the hospital this afternoon, telling reporters: 'I'll be back in a minute.' Kensington Palace said the baby was born at 11am, after Kate was taken by car before 6am to the Lindo Wing.
The Duchess of Cambridge will be leaving St Mary's Hospital this evening, the palace said, adding that George and Charlotte have returned to Kensington Palace after meeting their new brother.
William was present for the birth this morning, and both Kate and the child were 'doing well', according to a palace spokesman. Senior members of the royal family were all said to be 'delighted with the news'.
Royal fans celebrate this afternoon outside the Lindo Wing at St Mary's Hospital in Paddington, London
Royal fans toast the news that the Duchess of Cambridge has given birth to a baby boy, weighing 8lbs 7oz
Kensington Palace said: 'Her Royal Highness The Duchess of Cambridge will be leaving St. Mary's Hospital this evening. The Duke and Duchess of Cambridge and their son will travel home to Kensington Palace.'
Royal fans at Buckingham Palace gather for a glimpse of the easel announcing the birth of the royal baby today
A statement added: 'Prince George and Princess Charlotte have now returned to Kensington Palace. Their Royal Highnesses would like to thank all staff at the hospital for the care and treatment they have received.'
Royal watchers had been expecting a swift delivery at the Lindo Wing, with the child sharing a birthday with William Shakespeare and Lady Gabriella Windsor, the daughter of Prince and Princess Michael of Kent.
Sources told the Mail that George was at his school, Thomas's Battersea, today and was not taken out early. His sister Charlotte attends Willcocks Nursery in Kensington.
No other royal visitors are expected at the Lindo Wing today. It is anticipated that the rest of the family, including the proud grandparents, will meet their grandson at Kensington Palace tomorrow.
The self-proclaimed Royalist Town Crier announced the birth on the steps of the Lindo Wing, marking the moment by saying 'God save the Queen'. He drew cheers from the crowd of royal watchers outside the hospital.
An easel with a bulletin announcing the royal birth was put on display outside Buckingham Palace in London by footman Heather McDonald and and senior footman Olivia Smith.
The notice reads: 'Hey Royal Highness the Duchess of Cambridge was safely delivered of a son at 11.01am today. Hey Royal Highness and her child are both doing well.'
It will remain in place for around 24 hours. The practice of posting a bulletin announcing a royal birth has gone on for at least as long as Buckingham Palace has been the Sovereign's official residence - since 1837.
After it comes down, the notice will be sent to the Privy Council Office so it can be kept in their records.
A palace spokesman said: 'Her Royal Highness The Duchess of Cambridge was safely delivered of a son at 11.01am. The baby weighs 8lbs 7oz. The Duke of Cambridge was present for the birth.
'Her Royal Highness and her child are both doing well. The Queen, The Duke of Edinburgh, The Prince of Wales, The Duchess of Cornwall, Prince Harry and members of both families have been informed and are delighted with the news.'
The baby's sex was a surprise to William and Kate, who chose not to find out what they were having. The baby's title is His Royal Highness and he is a Prince of Cambridge.
Kensington Palace said the name of the baby would be announced in due course, and a formal notice of the birth would be posted on the forecourt of Buckingham Palace.
Celebrities and politicians have been sending their best wishes to The Duke and Duchess of Cambridge on social media.
Prime Minister Theresa May sent her 'warmest congratulations to the Duke and Duchess of Cambridge on the birth of their baby boy.' 'I wish them great happiness for the future,' she added.
Mayor of London Sadiq Khan tweeted: 'On behalf of Londoners, I want to send our warmest congratulations to the Duke & Duchess of Cambridge on the birth of their baby boy.'
Foreign Secretary Boris Johnson and Shadow Home Secretary Diane Abbott also sent their congratulations as politicians from across the political spectrum reacted with joy to the news of a new royal baby. Mr Johnson offered his 'huge congratulations to Their Royal Highnesses The Duke and Duchess of Cambridge on the safe delivery of their baby boy!'
Former world heavyweight champion boxer, Frank Bruno MBE, and actress Mia Farrow were among the celebrity well wishers on social media, while comedian David Walliams quipped: 'Prince Harry just got demoted again.'
The Archbishop of Canterbury and leader of the Liberal Democrats have also congratulated the royal family on social media.
Archbishop Justin Welby wrote: 'Congratulations to The Duke and Duchess of Cambridge on the safe delivery of their baby, a brother for Prince George and Princess Charlotte. May God bless them and all of their children with love, happiness and health.'
Sir Vince Cable, leader of the Lib Dems, tweeted: 'Congratulations to the Duke and Duchess of Cambridge on the birth of their son. I'm sure his older brother will be delighted he has been born on St George's Day! Best wishes to them all and Princess Charlotte.'
Speaker John Bercow told the Commons: 'I am sure the whole House would want to join me in sending Their Royal Highnesses, the Duke and Duchess of Cambridge, our warmest congratulations on the birth of their son.'
Defence Secretary Gavin Williamson added: 'I'd very much like to associate the Government with your comments and warm wishes.'
David Janzen, visiting the UK from Edmonton, Canada, was among those who gathered outside Buckingham Palace this afternoon.
'I was watching the news earlier and saw the announcement, so my wife and I decided to come down to Buckingham Palace and see it all happen,' said the 62-year-old.
Alain Ritzenthaler, from France, said the announcement was very different to his home country. 'We don't have a royal family like this, so it's exciting to see,' he said.
Natasha Wilburn did not know about the royal birth until she was stood outside the palace. 'I had no idea it was happening today,' said the 30-year-old from the US.
The senior medical team that looked after Kate during her labour were named by Kensington Palace, with Guy Thorpe-Beeston, surgeon gynaecologist to the royal household, and consultant gynaecologist Alan Farthing, surgeon gynaecologist to the Queen, joined by two familiar colleagues.
Dr Sunit Godambe, a consultant neonatologist at Imperial College NHS Trust, which runs St Mary's hospital, was part of the group that cared for the duchess during her previous two births alongside Mr Farthing and Mr Thorpe-Beeston.
Professor Huw Thomas, physician to the Queen and head of the medical household, completed the team. He was also involved with the birth of Princess Charlotte.
Earlier, excitement was building outside St Mary's Hospital, where some fans had been camped out for 15 days hoping for a glimpse of the royal baby.
The timing of today's announcement that Kate had gone into labour, falling on St George's Day, added to the occasion for waiting royalists outside.
John Loughrey, 63, who is part of a group who have been camped outside the hospital, said: 'We are so pleased. We have been here for 15 days.
'I'm so pleased it's St George's Day. St George himself would be very pleased if the baby's born today.'
Mr Loughrey, from Streatham, South London, said he plans to celebrate the birth with English flags and a portion of fish and chips.
'It doesn't matter if it's a boy or a girl as long as it's a healthy baby and a healthy mother,' he said. 'It's very good for our country and of course Her Majesty the Queen.'
Another member of his group, 82-year-old Terry Hutt, also welcomed the news.
'It's fantastic to have another newborn baby,' said Mr Hutt, who turns 83 at the end of the month and was hoping to share his birthday with the new royal.
'If they were born on my birthday, I would have got a birthday cake.'
Maria Scott, 46, from Newcastle, added: 'I think it's going to be a boy and I think it's going to be quick.'
The group were talking in front of their make-shift camp, including royal memorabilia and a Union Flag tent.
The Duchess is being cared for by consultant obstetrician Guy Thorpe-Beeston, who is the surgeon-gynaecologist to the household, and consultant gynaecologist Alan Farthing, the Queen's surgeon-gynaecologist - part of the trusted team who delivered George and Charlotte.
Highly-trained midwives were also on hand, while experts were waiting in the wings in case of an emergency.
For her previous births, Kate had a 23-strong team of top medics working or on stand-by from Imperial College Healthcare NHS Trust - which runs St Mary's Hospital.
Theatre staff were ready, along with a lab technician, replacement anaesthetists and paediatricians, a back-up for the consultant, and workers from a special baby care unit.
Today, press pens opened outside the hospital as journalists, photographers and camera crews waited for confirmation of the baby's arrival. Royal watchers were expecting a swift delivery.
With George, the duchess gave birth 10-and-a-half hours after being admitted to hospital, but Charlotte was born just two hours and 34 minutes after Kate arrived at the Lindo Wing.
The birth was declared in both conventional and contemporary ways.
An announcement was emailed to the press and a celebratory tweet posted on the Kensington Palace Twitter feed - @KensingtonRoyal.
There is also the traditional custom of placing a framed paper proclamation on an ornate gold stand behind the iron railings of Buckingham Palace.
Just like with her first and second children, Kate had been hoping for a natural birth and did not know whether she was having a boy or a girl.
Bookmakers were also waiting to see whether they have to pay out on the name, delivery date, hair colour and weight of the baby.
Predicted names include Alice, Alexandra, Elizabeth, Mary and Victoria for a girl and Arthur, Albert, Frederick, James and Philip for a boy.
George and Charlotte may visit the hospital to see their mother and baby sibling, just as George did when his younger sister was born three years ago.
But it depends how quickly Kate wants to head home.
While Kate was in labour, George and Charlotte were likely to be cared for by their full-time, live-in nanny, Maria Teresa Turrion Borrallo.
The duchess's pregnancy was announced by Kensington Palace on September 4 last year.
Kate again suffered from extreme pregnancy sickness hyperemesis gravidarum.
She went on maternity leave on March 22, but the duchess - and her large bump - attended an Easter Sunday church service in Windsor with the royals on April 1.
She has joked that William is 'in denial' about having a third child.
Although the Queen has four children, the duke is the first future king to have a third for more than 120 years.
In 1897, George V, then the Duke of York, welcomed a daughter - Princess Mary - the younger sister of Edward VIII and George VI.
Unlike William, George V never expected to be king and only became heir to throne when his older brother, the Duke of Clarence and Avondale, died from flu in 1892.