Royal baby boy: Kate, Duchess of Cambridge, and William show off new son

Kate and William have shown off their new baby boy to the world - "thrice the worry now', said William, while their son gave his first royal wave.

The Duchess of Cambridge and her husband left the Lindo Wing at St Mary's Hospital in London shortly before 5am (NZT), just seven hours after giving birth to her third child, weighing 8lb, 7oz.

A few hours earlier, during a photo call with his new son on the steps of the hospital, Prince William said Prince George and Princess Charlotte were "delighted" about the arrival of their little brother who was born at 11.01am (10.01pm, Monday, NZT).

The Royal couple posed for pictures with the Duchess wearing a red dress and holding her son who was nestled in a white blanket.

Towards the end of the Royal baby's first outing, the Duchess of Cambridge murmured to Prince William that she was worried about the baby getting cold before they headed back inside.

They then re-emerged with the baby in a carseat just before 5am NZT.

William joked that he didn't keep world waiting for too long after she went into labour at around 5pm Monday, NZT before giving birth five hours later.

He said of the name of the new baby: "You'll find out soon!" He also joked that he and Kate would have "Thrice worry now!"

As he left, the duke told reporters he was "very happy, very delighted, thank you".

The baby is fifth in line to the throne and the Queen and the Duke of Edinburgh's sixth great-grandchild.

The beaming parents looked delighted as they showed off their third son to the world who was born weighing 8lbs 7oz on St George's Day.

Earlier, the new baby's older siblings, Prince George and Princess Charlotte, were taken to meet him in their school uniform nearly six hours after Kate gave birth.

By the time the new baby was being shown off to the world, George and Charlotte had already returned to Kensington Palace.

In a statement, Kensington Palace said: "Their Royal Highnesses would like to thank all staff at the hospital for the care and treatment they have received.

"They would also like to thank everyone for their warm wishes."

Daily Mail