- Video report by ITV News Royal Editor Chris Ship
The Duke and Duchess have introduced their newborn son, weighing 8lbs 7oz, to the world.
The happy parents stood outside St Mary's Hospital and waved to crowds eager to see the latest addition to the Royal family.
As Prince William moved to take his new son home, he remarked there would be "three times more to worry about."
The baby prince, fifth in line to the throne and Kate's third child, was safely delivered at 11:01hrs on patriotic St George's Day.
The palace says the Queen, the royal family and Kate's family are delighted with the news, and mother and baby are both doing well.
Shortly after 4pm, a beaming Prince William, who had been at Kate's side for the birth, emerged from the hospital.
Cheered by onlookers he return to Kensington Palace to collect Prince George and Princess Charlotte and bring them to meet their new brother.
They were back within minutes. Both children looked slightly over-awed by the many journalists and well wishers.
But then Charlotte turned and melted hearts by waving to the crowd.
Their visit was brief, but the Duchess of Cambridge will herself return home today, Kensington Palace said.
Kensington Palace has 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."
Kate had been taken to the Lindo Wing of St Mary's Hospital in Paddington some time ahead of 6am this morning.
As the news spread, the media quickly assembled outside the hospital.
Already waiting were some excited fans who had been camping outside the hospital for the last 15 days to ensure they caught a glimpse of the royals.
Following the official announcement, the self-proclaimed Royalist Town Crier made sure those who had gathered outside the Lindo wing had been made aware that a royal child had been born.
He drew cheers from the crowd of royal watchers as he mounted the steps outside the hospital and marked the moment by shouting: "God save the Queen".
As is traditional, Buckingham Palace placed a framed notice of the birth on a ceremonial easel on the Palace forecourt, where it will remain for about 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.
Prime Minister Theresa May issued her best wishes.
"My 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 said.
Archbishop Justin Welby wrote on Twitter: "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."
Bookmakers will be following events carefully to see if they have to pay out, including over the baby's name.
Kensington Palace said the baby's name would be "announced in due course", but Arthur is already the favourite with punters, according to the bookies.
The Duchess's pregnancy was announced by Kensington Palace back on September 4 last year.
Kate, who again suffered from extreme pregnancy sickness hyperemesis gravidarum, 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 is known to have 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.