The world's media waited in the summer heat outside the private London hospital on Saturday where Prince William's wife Catherine will give birth, as the expected due date arrived with no sign of the royal heir.

A rumor that the Duchess of Cambridge had gone into labor on Thursday spread like wildfire on Twitter and reportedly caused Prime Minister David Cameron's office to call Buckingham Palace to check on it.

It was another false alarm, but the dozens of international journalists camped outside St Mary's Hospital are on tenterhooks, knowing that it could happen any day now.

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The palace has said the baby was due in "mid-July" and many editors have had this weekend in the diary for months -- even though any parent knows that babies rarely arrive on time.

William's father, Prince Charles, revealed his anticipation as he attended a festival celebrating Queen Elizabeth II's coronation on Friday.

The heir to the throne said "it won't be long now" until he becomes a grandfather as he surveyed a range of commemorative china to mark the new arrival -- one of countless ranges of memorabilia celebrating the birth.

His second wife Camilla added: "We are very excited. Immensely looking forward to it and waiting for the phone call."

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The popularity of William and Kate, who married in a glittering wedding at Westminster Abbey in 2011, has turned the birth of their first child into a global event.

Media organizations from around the world have been installed outside the hospital for almost two weeks now.

In the absence of news, time has been passing slowly. But at least the famous British rain has held off, with Saturday tipped to be the hottest day of the year so far.

For the television networks, the top priority is to hold their positions around the clock, working 12-hour shifts.

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That means fiercely defending their territory, never yielding an inch of space to a rival station, and woe betide anyone touching the gaffer tape marking out an organization's patch.

The main British news broadcasters -- BBC, ITN and Sky News -- have got the prime spots, lined up in front of the major US networks, which have maximized their space with some mammoth pieces of broadcasting hardware.

Behind them, it is a scramble to get a decent angle to shoot the doorway where William himself was first presented to the world in 1982, carried out of the Lindo Wing by his parents Prince Charles and Diana.

An bus drives past St Mary's Hospital where Catherine, Duchess of Cambridge is due to give birth.Corbis

For the time being, the door is guarded by a police officer who is rapidly becoming the most filmed man on the planet.

Occasionally he breaks his vigil to let a pregnant woman into the building, as the hospital continues the day-to-day business of treating less high-profile patients.

International correspondents pad out the time by interviewing passers-by and, as a last resort, one another.

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Shipped in from Belgium, Christophe Giltay, a senior reporter from RTL-TVI, is "following it all from a distance".

"You take in the ambiance and see how your colleagues are getting along. It's a right royal free-for-all," he said.

He drew parallels with the media scrum outside the Pretoria hospital where the frail former South African president Nelson Mandela is being treated.

In the knowledge that "the world has pretty much covered it all already", the Belgian tries to "find original angles".

"We're looking. Maybe something on the name. Yes, we're looking," he said, before scaling a metal ladder to deliver his report.

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From high up in a forest of aluminum, he has a good view of his Polish colleague trying for the 10th time to secure a moving shot in front of the hospital.

After nearly being run over by a passing car, the Pole finally captures it and can loosen his tie in the heat. "What a mess! But we did it," he said.

A hundred meters away, the satellite vans are parked up, ready to whirr into life at any moment.