Kate, Duchess of Cambridge, leaves hospital with third child

The couple smiled and waved to cheering crowds before returning inside the hospital. A few minutes later, they departed for home. Prince William carried the baby in a baby capsule and held Kate's hand as they walked down the stairs.

The name of the baby, who is fifth in line to the throne, has not been announced.
Photo: AP

Putting the baby in the back seat, the beaming prince was asked by reporters how he felt.

"Very happy, very delighted, thank you," he said, before joking "thrice the worry now" and holding up three fingers.

It was the second drive home for Prince William. Earlier he left to collect the new baby's older siblings Charlotte and George.

Arriving back at the hospital just before 5pm, Charlotte - who is about to turn three - waved eagerly to the wall of reporters, photographers and television cameras.

Prince George and Princess Charlotte arrive to meet their new baby brother.
Photo: AP

Four-year old George, dressed in his school uniform, appeared overwhelmed by the spectacle and kept his gaze down. Holding their hands, Prince William ushered them into the hospital to see their mother and meet their new baby brother. They were privately whisked back to Kensington Palace.

The couple have not announced the name of the prince, who is now fifth in line to the British throne. His arrival bumps his uncle Prince Harry down to sixth in line.

The royal baby arrived on St George's Day, which honours England's patron saint and came two days after the Queen's 92nd birthday. It is her sixth great-grandchild and she is the first person who was notified of the birth.

Kate, Duchess of Cambridge, appeared before well-wishers just six hours after giving birth.
Photo: AP

"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," Kensington Palace said.

As with the birth of Prince George and Princess Charlotte, Kate's labour was swift, meaning the hundreds of well-wishers gathered outside the hospital only had a relatively wait for their first glimpse of the prince.

Many dressed in or carrying Union Jacks and had gathered outside the hospital when the palace announced the Duchess had gone into labour just before 8.30am.

Royalists' hopes that Kate would leave hospital on the same day, as she did when she gave birth to Charlotte, were heightened when the Duchesses' stylist was spotted leaving the hospital around lunch-time.

As with her earlier pregnancies, Kate suffered from hyperemesis gravidarum, or severe morning sickness.

Well-wishers wait outside the hospital hoping to catch a glimpse of the baby prince.
Photo: AP

The Duchess' medical team included Guy Thorpe-Beeston, the royal household's surgeon gynaecologist, the Queen's surgeon gynaecologist Alan Farthing, Dr Sunit Godambe, a neonatologist, and Professor Huw Thomas, who serves as physician to the Queen.

A large Union Jack flag was raised over Buckingham Palace and a State Standard over Windsor Castle.

Self-styled town crier Tony Appleton announces the royal birthoutside the Lindo wing at St Mary's Hospital in London.
Photo: AP

The government ordered flags on UK government buildings be flown at full mast to mark the royal birth.

Prime Minister Theresa May tweeted her congratulations.

"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," the prime minister said.

Foreign Secretary Boris Johnson was among the first government figures to issue his congratulations.

"Huge congratulations to Their Royal Highnesses The Duke and Duchess of Cambridge on the safe delivery of their baby boy today!" Mr Johnson tweeted.

Australia's Governor-General Sir Peter Cosgrove led the congratulations on behalf of Australians.

"Lynne and I send you our warmest congratulations on the birth of a new Royal prince," he said.

"As you already know the arrival of a new baby is another exciting and life-affirming event you no doubt will cherish,

"On behalf all Australians, we send your family our best wishes at this happy and special time."

Latika Bourke is a reporter for The Sydney Morning Herald and The Age based in London. She has previously worked for Fairfax Media, the ABC and 2UE in Canberra. Latika won the Walkley Award for Young Australian Journalist of the Year in 2010.