* Kate taken to London hospital in labour before 6 a.m.
* Baby will be third-in-line to throne
* Birth seals comeback in royal fortunes
Prince William's wife Kate was in labour at a London hospital on Monday as public anticipation over the birth of their first child, who will be third in line to the British throne, reached fever pitch.
The royal couple, officially known as the Duke and Duchess of Cambridge, travelled by car to St. Mary's Hospital in west London before dawn and entered through a back door to avoid media camped outside the main entrance.
Kate is due to give birth to the child at the private Lindo wing of the hospital, where her husband was born to the late Princess Diana in 1982.
Kate and William, both aged 31, met when they were students at St. Andrews University and were married in April 2011 in a spectacular wedding broadcast around the world.
The royal birth has provoked a similar frenzy, with national and international media keeping up a deluge of speculative reports throughout Monday from outside the hospital.
The birth will be announced in the traditional way with an envelope containing the baby's details taken from the hospital to Queen Elizabeth's London residence, Buckingham Palace, where the news will be posted on a board outside the main gates.
"Things are progressing as normal. It wasn't an emergency," a royal spokeswoman said.
Prime Minister David Cameron said it was an exciting time.
"Best wishes to them, a very exciting occasion and the whole country is excited with them. Everyone's hoping for the best," Cameron told the BBC.
Crowds were also gathered outside Buckingham Palace.
"We love anything and everything about the royal family. As Canadians we really respect the queen," said Rosch Neboulsi, a tourist from Alberta, Canada, who was standing outside the palace gates with his family.
The baby will arrive at a time when the royal family is riding a wave of popularity. An Ipsos Mori poll last week showed 77 percent of Britons were in favour of remaining a monarchy over a republic, close to its best-ever level of support.
The event seals a remarkable comeback for the House of Windsor, regenerated by the younger royals. The death of Princess Diana in 1997 had led to a dip in popularity amid accusations it was out of touch with modern Britain.
But last year's celebrations of Queen Elizabeth's 60th year on the throne showed the affection with which she is held by most of the population, despite a small but vocal Republican movement.
The royal birth might also help raise the national mood, battered by economic problems, unemployment, cuts in public spending and a rising gap between rich and poor.
The festive atmosphere has been heightened by a heatwave and a string of British sporting victories in tennis, rugby, cricket and cycling.
Royal supporter Terry Hutt, 78, who has waited outside the hospital for 12 days, acknowledged his joy over the imminent arrival.
"We've got a lovely married couple and baby will make three and they will be a family. It means everything to me, girl or boy, as they will be king or queen one day," said Hutt, dressed in a suit and hat emblazoned with Union Jacks.
The royal baby was also dominating headlines globally.
"You'd think it was the American royal family producing this baby," said Robert Lacey, royal historian and biographer.
Royal sources said Kate has planned a natural birth with William, a Royal Air Force search and rescue helicopter pilot, to be at her side. The royal couple do not know the baby's sex.
The child will be third in line to the throne regardless of gender behind grandfather Prince Charles and father William after Britain and 15 other Commonwealth nations agreed to change royal succession rules so males no longer take precedence.
"Fundamentally all of them have agreed in writing to this," Cameron said. "It would not be a problem."
Royal officials have confirmed the baby will be known as His or Her Highness Prince or Princess (name) of Cambridge. The name may not be announced immediately - it took more than a week for an announcement of William's name.
Bookmakers have a girl as the favourite with preferred names Alexandra, Victoria, Charlotte and Diana, in honour of William's mother, who died in a car crash in Paris in 1997. George is the favourite boy's name followed by James.
Queen Elizabeth will be among the first to be informed of the arrival of the baby, who will be delivered by Marcus Setchell, the queen's former gynaecologist.
After the birth, commentators said Kate was expected to spend time at her parents' house in the village of Bucklebury, about 50 miles (80 km) west of London, before eventually moving to London's Kensington Palace, William's childhood home.
Kate, whose ancestors were coal miners, has become hugely popular and a fashion icon, with her attire scrutinised and copied every time she steps out in public.
Scrutiny of Princess Diana's life and her death while pursued by paparazzi instilled a deep distaste for the media in William and he has done his best to shield his wife from such attention.
Analysts said any economic impact from the royal birth would be positive but limited, with no public holiday and purchases of souvenirs or alcohol to toast the baby likely to be the main boosts.
"Having said that there is a lot of international interest in the royal baby with high foreign media coverage, which does help to advertise the UK globally," said Howard Archer, economist from IHS Global Insight.
Not all Britons, however, were giddy over the event.
The website of the Guardian newspaper offered readers the option of clicking on a "Republican" prompt, removing all references to the royal birth from its pages.
The Republic movement said on its website that every child should be born equal.
"How can that be when one child is born above all others, destined for high office not because of merit or popular choice but because of their parents?" it said.
($1 = 0.6386 British pounds) (Additional reporting by Belinda Goldsmith, Sarah Young, Limei Hoang, and Mark Anderson,; Editing by Angus MacSwan)