William said he and his wife "could not be happier" over the new arrival, who weighed in at 8 pounds, 6 ounces (3.8kg) at St Mary's Hospital in central London. British politicians and leaders the world over, including Barack Obama, sent their congratulations. London mayor Boris Johnson announced that the fountains of Trafalgar Square would run with blue water for the next week to mark the event.

In a sign of the century that the royal baby has been born into, announcement of the birth was made to the world first via e-mail and social media, ahead of the traditional method of posting the news on a piece of paper mounted on an easel at Buckingham Palace. The language was still archaic - almost biblical - with its proclamation that Catherine, the Duchess of Cambridge, "was safely delivered of a son" at 4.24 pm.

"It is an incredibly special moment for William and Catherine, and we are so thrilled for them on the birth of their baby boy," Prince Charles said. "I am enormously proud and happy to be a grandfather for the first time, and we are eagerly looking forward to seeing the baby in the near future."

Word of the birth capped weeks of international media frenzy, with photographers and journalists staking out the hospital around the clock for the briefest of glimpses of the pregnant duchess being whisked inside.

Speculation surrounding the unborn baby's gender now shifts to his name. It took a week for William's name to be unveiled after his birth in the same hospital in 1982. The bookmakers' favourite choices for the new "prince of Cambridge," the child's official title, are George and James, with 500-1 odds on the name Hashtag.

The royal household is riding a wave of popularity not seen since the heyday of William's late mother, Diana, Princess of Wales. The queen's diamond jubilee last year, and William and Kate's wedding in 2011, have allowed the monarchy to promote itself as both something old and something new, an ancient institution with a modern, youthful and good-looking face.

A few public missteps and naked Harry photos notwithstanding, the dark days that descended on the monarchy after the shocking death of Diana in a 1997 car crash are mostly a distant memory.

"This is the first time we've had a new king or queen born in our generation," said Holly Appleton, 27, who joined the crowds of people thronging the plaza in front of Buckingham Palace after the birth was announced. "It adds something to the party."

Susan and Chuck Johnson, retired teachers visiting England for the first time from South Carolina, rushed over to Buckingham Palace when they heard the news of the birth. Susan could relate - on Monday, she turned 65.

"It's my birthday today so I'm really the princess," she said with a laugh.

"We thought it'd be fun to be here. You can say: 'I was here when,'" her husband added, watching appreciatively as the guards in their red uniforms and bushy black hats performed in the palace forecourt. "They do ceremony well."

The national hoopla over what is, for most Britons, the child of total strangers offered a distraction from their fitful economy, their political scandals and their own screaming kids. Already buoyed by triumphs in the Tour de France and at Wimbledon, many raised a glass Monday evening to toast the new monarch-to-be, or perhaps just to beat the heat on the hottest day of the year, with the mercury soaring past 90 degrees in some parts of London.

, Britain's bestselling tabloid, changed its front-page masthead for Tuesday's editions to read: " ."

There are now three generations of direct heirs to the throne, aged 64 (Charles), 31 (William) and 0 (the newborn), a rare pile-up in the annals of British royalty. The last time the country experienced such an heir supply was during Queen Victoria's reign more than 100 years ago.

Exactly when the oldest of them, Charles, will finally become king is anyone's guess, since his 87-year-old mother regards her position as a divine appointment to be curtailed only by death or incapacitation, not voluntary abdication.

A baby girl would have made an extra bit of history by being the first one to be guaranteed succession even if she had brothers. A new rule being ratified by the 16 "realms" removes the automatic right of male offspring to leapfrog over older sisters on to the throne, a discriminatory formula that held for centuries.

"I was really hoping it would be a girl, because then it would be our next queen," said a slightly disappointed Charissa Coulthard, 27.

The duchess, 31, was attended by the queen's own gynaecologist, part of her top-notch medical team at St Mary's Hospital, where delivery of a child costs upward of US$7,500. Fortunately for Kate's privacy, the royal family no longer submits to the traditional indignity of having the archbishop of Canterbury and a government cabinet secretary present for the birth to attest that the baby is a genuine heir and not an impostor smuggled in.

William was by his wife's side, palace officials said, just as his father was in the room when he was born. By contrast, Prince Philip was off playing squash when Charles was delivered in Buckingham Palace.

In another up-to-date touch, William will take two weeks of paternity leave from his job as a military helicopter pilot. He and Kate are expected to be more involved in their child's upbringing than previous generations of royals were.

"This is going to be the first royal child that's going to be raised without a team of nannies, without royal footmen," said Ingrid Seward, editor of the monthly magazine . "People all over the world will be watching as to how they raise this child, because they're celebrities, aren't they? They have a following."

How intrusive the paparazzi will be remains to be seen. Mindful of his mother's difficult relationship with the media, whom he blames for chasing her to her death in a Paris tunnel, William has been protective of his wife, especially during her pregnancy, when her public appearances were considerably reduced.

They were able to avoid the media Monday, entering the hospital through a side entrance away from the cameras about 6 am. Official confirmation of her presence inside came more than an hour later. Likewise, the palace announced the birth four hours after the fact, following notification of the queen and senior members of the royal household. Merchants are rubbing their hands over another mass-marketing opportunity, with estimates that the royal birth could be worth more than US$350 million for Britain's battered economy. That could help compensate for the lost productivity from the national holidays declared for the queen's jubilee and William and Kate's wedding.