The Queen appeared in a black cab outside St Mary's Hospital in London, where Kate Middleton has just given birth to her third child.

Only it wasn't really Her Majesty, but a lifelike waxwork mannequin dressed in a pink jacket and pearls.

The lifelike wax figure turned a few heads as it was driven past the hospital where Catherine, Duchess of Cambridge, was giving birth to her third child. Photo / AP
The lifelike wax figure turned a few heads as it was driven past the hospital where Catherine, Duchess of Cambridge, was giving birth to her third child. Photo / AP

The crowd of royal fans waiting outside the Paddington hospital found the bizarre prank "very funny", according to onlookers. The black cab attracted instant attention and many rushed to the door for a look inside, before having to be told "it's wax".

It wasn't immediately clear who had planned the joke, as media and well-wishers waited for a sighting of Kate and the baby boy after Prince William left the Lindo wing promising to be "back in a minute".

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The Duke of Cambridge left the hospital around 4pm, London time, before returning with Prince George, 4, and Princess Charlotte, 2, in accordance with tradition.

Outside the Lindo wing, royal superfans who had been camped out for two weeks mixed with curious tourists and hospital staff trying to make their way through the madness.

John Loughrey, who has also attended the engagement announcement of Prince Harry and Meghan Markle in November, told news.com.au he was "going to celebrate with fish and chips".

A notice is placed on an easel announcing the birth of the new Royal baby, in the forecourt of Buckingham Palace, in London. Photo / AP
A notice is placed on an easel announcing the birth of the new Royal baby, in the forecourt of Buckingham Palace, in London. Photo / AP

Maria Scott of Newcastle, had been outside the hospital for more than two weeks and said: "We're very proud of our monarchy".

"It's really important to show support because they need to know how much they are loved by the people," she said.

The "town crier" said the "name will be Philip, I promise you", while others speculated on Albert or Arthur.

Ladbrokes spokesman Alex Apati said "customers think they're onto something with Arthur" which has become a punters favourite recently. The agency also favours traditional royal names like Philip, Frederick, and James.

"History and tradition kind of says that the names are very straight down the line, nothing too out there.

"A gender neutral name? Alex maybe?" he said.

Friends Onkar and Sukhdev Singh said they were "really excited" hearing the news and jumped on a train from West London to wait outside the hospital in the hope of catching a glimpse of the young Prince and Princess.

"It's such an amazing day in history in that it's the first time a woman has been ahead of a man in succession to the throne," Onkar said, referring to the fact Charlotte will not be bumped down the line because she is female.

"So that makes a woman fourth in line for the throne after today. We can all celebrate that as British people."

Britain's Prince William arrives with Prince George and Princess Charlotte at the Lindo wing at St Mary's Hospital. Photo / AP
Britain's Prince William arrives with Prince George and Princess Charlotte at the Lindo wing at St Mary's Hospital. Photo / AP

The Duchess of Cambridge delivered the baby at 11.01am local time, about three hours after she arrived at the hospital by car.

Although some are perplexed by the royal fever sweeping the world, others said the world needed happy news, as the story began trending on Twitter.

Comedian David Walliams joked that Prince Harry "just got demoted again", as the newborn is now fifth in line to the throne, bumping his uncle to sixth place.

Kensington Palace announced the news online and via a noticeboard outside Buckingham Palace, which will stay there for 24 hours.

"Her Royal Highness The Duchess of Cambridge was safely delivered of a son at 1101hrs. The baby weighs 8 lbs 7oz [3.8kg]," the palace said.

"The Duke of Cambridge was present for the birth. 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.

"Her Royal Highness and her child are both doing well."