Image caption The duchess's due date has not been officially announced

The Duchess of Cambridge is in labour, as the world's media gather outside the hospital awaiting news of the latest addition to the Royal Family.

Catherine and the Duke of Cambridge arrived by car at St Mary's Hospital in Paddington, west London, at 06:00 BST.

Kensington Palace said she was in the early stages of labour, which was progressing as normal.

The couple do not know the sex of their first child, who will be third in line to the throne.

The world's media have been camped outside St Mary's for days in anticipation of the birth. The due date had never been officially announced but had widely been expected to be mid-July.

Royal vehicles were seen at a back entrance to the hospital at about 06:00 BST, with the announcement coming from Kensington Palace 90 minutes later.

Police presence

The couple, both 31, travelled to the hospital without a police escort, their spokesman said.

Analysis The statement from Kensington Palace contains just 45 words. Such is the global fascination with this baby, those words will be translated into countless languages and endlessly repeated until fresh information is provided once the child has been born. This intense international interest highlights the challenges facing William and Catherine. The birth of a child is a deeply personal and private moment. But in their case, it will also be the cause of much wider celebration. Prince Charles had a similar experience when Prince William was born. He wrote of being rewarded by seeing "a small creature" who "belonged to us even though he seemed to belong to everyone else as well". Thirty-one years on, the world waits while the Duchess of Cambridge is in labour - something which is not without risk. Her baby will represent the beginning of family life for the couple and will offer the House of Windsor renewed hope for the future.

The duchess is being tended by a top medical team led by the Queen's gynaecologist Marcus Setchell, who delivered the Countess of Wessex's two children. Alan Farthing, gynaecologist to the Royal Household, will be assisting.

She will give birth in the hospital's private Lindo Wing, where Prince William and his brother Prince Harry were born.

There is a strong police presence around the hospital and two police officers are guarding the entrance to the private wing. The media scrum parked outside is having to contend with the heat on what is the hottest day in the UK since July 2006.

BBC royal correspondent Peter Hunt said the duchess was thought to be one week past her due date.

"The royal statement contains 45 words. All there will be to sustain the world's media until the baby is born," he tweeted.

Last week, the Queen joked about the imminent arrival of the newest member of her family, saying she hoped the baby would be born before she went on holiday to her private Balmoral estate in Scotland.

During a visit to the National Railway Museum in York, the Prince of Wales was asked by the members of the press and the public about the birth of his first grandchild but he did not divulge any information.

Easel announcement

Prince William has been with his wife on annual leave and will have two weeks' paternity leave from his job as an RAF search and rescue helicopter pilot.

It is believed the next time the couple will be seen in public is when they appear on the steps of the hospital with their baby.

Image caption Crowds gathered on Monday morning for the changing of the guard at Buckingham Palace, with many interested in news of the royal baby Image caption Members of the world's media had been camped outside St Mary's Hospital for weeks Image caption The crowds outside the hospital grew steadily through the morning as journalists and royal watchers gathered to await further news Image caption Broadcasters sprung into action early on Monday after Kensington Palace announced the Duchess of Cambridge was in the early stages of labour Image caption Journalists were not the only ones waiting outside the Lindo Wing. Royal supporter Terry Hutt has been camped outside for nearly two weeks Image caption Tourists also gathered outside the hospital after news broke that the third in line to the throne was on the way Image caption The Duke and Duchess of Cambridge were married in April 2011 Image caption The announcement that Catherine was pregnant came in December when she was admitted to hospital with acute morning sickness Image caption Catherine made her final public appearance at celebrations for the Queen's official birthday on 15 June previous slide next slide

Plans for how the birth will be announced were amended by Kensington Palace officials on Monday evening.

A formal press release containing details of the birth will now be issued shortly before a formal signed bulletin leaves the hospital for Buckingham Palace.

It is custom for news of royal births and deaths to be attached to the railings of the palace, but in this case it will be displayed on an ornate easel in the forecourt of the palace.

The Queen, senior members of the Royal Family and the duchess's family - if they are not at the hospital - will be told about the birth first.

Then a royal aide will take a bulletin, signed by key medical staff, from the hospital to the palace under police escort.

Media playback is unsupported on your device Media caption The Duke and Duchess of Cambridge's first child will be third in line to the British throne

Under new laws to succession the baby will remain third in line to the throne regardless of gender and the gender of any subsequent siblings.

Prime Minister David Cameron spoke about the excitement surrounding the imminent arrival, and also about the recent changes to the rules of succession.

Speaking on BBC Radio 4's Woman's Hour, he said it was "pretty much a done deal".

'Thoughts and prayers'

All the nations of the Commonwealth have agreed, he said, even if the changes had not yet passed through their parliaments.

The Archbishop of Canterbury, who is likely to carry out any christening, has sent his best wishes to the couple.

"My thoughts and prayers are with Kate and the whole family on this enormously special day," tweeted the Most Reverend Justin Welby.

How will the birth be announced? An official announcement signed by key medical staff will be driven to Buckingham Palace under police escort

It will be displayed on an ornate easel in the palace forecourt

The gender and weight are likely to be included in the announcement, but the name is not expected to be revealed straight away

An announcement will then be posted on Twitter and Facebook, and the media will be informed 10 curious things about the royal birth How royal births have been announced

There is global interest in the royal birth and it has been the subject of much speculation, particularly when it comes to names.

Bookmaker William Hill says George is the favourite for a boy and, after a flurry of bets, Alexandra - one of the Queen's middle names - for a girl. Charlotte is also thought to be a contender.

The new royal baby will be the Queen's third great-grandchild. It will be expected to take the throne following reigns by Prince Charles and then Prince William.

The couple announced they were expecting their first child in December after the duchess was admitted to hospital suffering from severe morning sickness.

Since the announcement, the duchess has carried out 19 days of public engagements before going on maternity leave in the middle of June.

Catherine's final public appearance before the birth was at the Trooping the Colour ceremony in June.

The Duke and Duchess of Cambridge were married at Westminster Abbey in April 2011, watched on television by hundreds of millions of people around the world.