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
Fans 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 Duchess of Cambridge has today given birth to her third child - a boy weighing 8lbs 7oz - on St George's Day.
Kensington Palace announced that the baby was born at about 11am, after Kate was taken by car before 6am to the private Lindo wing of St Mary's Hospital in Paddington, London.
The birth appears to have taken place without complications. The baby is fifth in line to the throne, the Queen and the Duke of Edinburgh's sixth great-grandchild and a younger brother for Prince George and Princess Charlotte.
Prince William was present for the birth, and both Kate and the child were 'doing well', according to a palace spokesman. Senior members of the royal family were said to be 'delighted with the news'.
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.
The Duchess of Cambridge has given birth to a baby boy. The couple are pictured on March 22 in Stratford, East London
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
Supporters of the royal family celebrate outside the Lindo Wing following the news of the royal birth
One man drinks from a bottle as royal fans celebrate outside the Lindo Wing after the news from Kensington Palace
A town crier stands outside the Lindo Wing of St Mary's Hospital in Paddington after the birth was announced today
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.
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.
Royal fans John Loughrey (right) and Terry Hutt (left) pose for a photo opposite the Lindo Wing at St Mary's Hospital today
Royal fan Sharon McEwan is camped today outside the Lindo Wing at St Mary's Hospital in Paddington, London
Royal fans John Loughrey (left) and Terry Hutt (right) hold dolls outside the Lindo Wing at St Mary's Hospital this morning
Supporters of the royal family stand outside the Lindo Wing of St Mary's Hospital in Paddington, London this morning
An unsuspecting couple emerged from the Lindo Wing with their newborn baby to be greeted by the world's media. They posed for photographs outside the famous door in front of dozens of journalists and photographers before leaving in a taxi
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.'
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.'
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.'
Members of the media gather outside the Lindo Wing today after Kensington Palace announced Kate had gone into labour
Royal fans are interviewed by journalists outside the Lindo Wing at St Mary's Hospital in Paddington this morning
Royal fans stand this morning outside the Lindo Wing, where the Duchess of Cambridge was admitted after going into labour
Well wishes hold flags outside the Lindo Wing this morning as they await news of the new royal baby
The official announcements that Kate had gone into labour - and given birth - were released by Kensington Palace today
The Duke of Duchess of Cambridge pose with Princess Charlotte outside the Lindo Wing following her birth in May 2015
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.