LONDON (AFP) - Could this be the week? Britain's royal family and the world's media are on tenterhooks awaiting the birth of Prince William and wife Catherine's first child, a baby who will one day be king or queen of Britain and a diverse group of commonwealth countries.

Millions of people worldwide watched Prince William's glittering televised marriage to Kate Middleton in 2011 and, judging by the media camped outside her London hospital, their baby's arrival will be a similar global sensation.

The baby is not officially due until mid-July but this hasn't stopped news organisations from setting up dozens of camera positions outside the private Lindo Wing of St Mary's Hospital, where Prince William was born in 1982 and his brother Harry in 1984.

The palace is also making preparations.

A privately charted helicopter is reportedly on standby to whisk Prince William, a Royal Air Force search-and-rescue helicopter pilot based in Wales, to his wife's side in what British newspapers on Sunday dubbed "Operation heir-lift".

The royal couple, both 31, insist they do not know whether their firstborn is a girl or boy - but in either case the baby will be third in line to the throne after 64-year-old Prince Charles and his son William, pushing William's younger brother Harry into fourth place.

And if it is a girl she will be the first princess in British history who cannot be pushed down the line of succession by any younger brothers, following a momentous change in the law.

Bookmakers say punters heavily back a female child, with Alexandra and Charlotte the top picks for girls' names and James and George for a boy.

The baby's arrival comes as the royal family enjoys record popularity after a difficult few decades, and highlights the British monarchy's enduring strength even as its European counterparts undergo fundamental changes.

Huge crowds thronged London's streets for William and Kate's wedding in April 2011, and again in 2012 for the diamond jubilee celebrations marking Queen Elizabeth II's 60th year on the throne.

The feelgood factor of a new royal baby, delivering a much-needed public morale boost at a time of economic gloom, is likely to increase the royals' current popularity even further.

But public support is volatile, warns Patrick Jephson, former chief of staff to Prince William's late mother Princess Diana.

"The bad times, that will be the test," he told AFP.

There was deep public ill-will towards the monarchy following Princess Diana's death in a 1997 car crash. Devastated Britons saw the royal family as cold and out-of-touch, and it took years to rehabilitate their reputation.

These days the queen has an approval rating of 84 percent, according to poll by YouGov last month, and the palace has worked hard to portray William and Kate as a modern young couple with lives not so different from those of their subjects.

Kate is the first "commoner" to marry a future king since 1660 - her parents may be rich but her mother was an airline stewardess and they made their money themselves through a party supplies business.

The royal couple have led a relatively simple married life until now at a farmhouse in Wales close to Prince William's airbase, though after the birth they will move into London's Kensington Palace, which has been refurbished at a cost of £1 million (S$1.9 million) of British taxpayers' money.

No childhood inside palace walls can be completely normal, but the couple are believed to be seeking a "hands-on" role as parents with minimal help from nannies.

Once the baby is born, a proclamation signed by Princess Kate's doctors will be rushed to Buckingham Palace and displayed on a golden easel at the gates, though in a nod to the social media era the birth will also be announced on Twitter and Facebook.

Later, cannon fire will echo over the skies of London to hail the new arrival.

The new royal will be prepared for a lifetime of duty.

He or she will one day be head of state in Britain and the commonwealth realms which share the British monarch - there are 15 at present including Canada, Australia and New Zealand - but with both Charles and William set to take their turns at being king first, it could be a long wait.

At 87, Queen Elizabeth II shows no sign of following in the footsteps of the monarchs of Belgium, the Netherlands and the Gulf state of Qatar, who have all decided in recent months to hand over power to younger heirs.

"Charles will have spent the whole of his adult life in waiting and in training for a role that he doesn't assume until he's very old. I don't think it's kind to our monarchs, nor is it kind to those next in line," said Robert Hazell, director of the Constitution Unit at University College London.