The public and the world's media are waiting for official news of the Royal baby after the Duchess of Cambridge went into labour.
Doctors say the Royal Family could have its newest member by midnight - the average length of labour for a first-time mother is 12 hours.
Sky News' Royal correspondent Paul Harrison said an announcement after dusk would be unlikely.
"When it gets dark around 10.30pm the whole theatre of someone emerging form the Lindo wing carrying a notice of birth probably won't happen.
"It's unlikely Kensington Palace would wake the Queen at that time if she is asleep. If she can't be told it's unlikely we’ll get news tonight".
Kensington Palace has announced that a formal press release will be issued via email with the news, instead of by placing a notice on an easel outside Buckingham Palace.
If this happens it will be detract from the tradition of a Palace official making an announcement on the steps of the hospital before taking the notice by car to the Palace.
The brief foolscap-sized note on headed Palace notepaper, traditionally confirms the sex of the baby but usually gives away little else other than that the baby has been "safely delivered" and perhaps the weight.
Although the notice will still be placed on the easel, it won't be as significant as the news will already have been announced.
Prince Charles, who is on a two-day visit to Yorkshire, told Sky News at York's National Railway Museum earlier that he knew "absolutely nothing" about his first grandchild's impending arrival.
At York Minster, members of the public shouted "Congratulations" to Charles, who walked over to one woman, smiling, and said: "Do you know something I don't?"
At Harewood House the Prince told a reception for supporters of the Prince's Trust how "approaching grandfatherhood" was "taking a certain amount of time in coming."
In a speech he thanked Richard Jackson, the trust's development committee chair in Yorkshire.
He said: "I'm very grateful to Richard Jackson for mentioning the approaching onset of grandfatherhood which is taking a certain amount of time in coming."
Charles then said a visit to an abbatoir today reminded him he was approaching pensionable age and being a grandfather at about the same time.
Some 250 reporters and crew are packed into a relatively small stretch of the street opposite the hospital, with hundreds of photographers lenses poised to capture the first images of the new heir.
During the day, Royal staff have been sending personal possessions and supplies to the hospital where the Duchess of Cambridge is in labour.
Kate, who is thought to be more than a week past her due date, was taken through a side entrance of St Mary's Hospital in London just before 6am.
The Duke of Cambridge travelled with her by car from Kensington Palace, where she went into labour naturally after spending the weekend there.
She plans to have a natural birth at the hospital's private Lindo wing and the couple's spokesman said: "Things are progressing as normal."
Sky News' Royal Correspondent Paul Harrison said it was believed a number of "bits and bobs" had been brought to the Duchess by staff via side entrances at around lunchtime.
It also appears officials staged a "dummy run" in preparation for her arrival amid reports of police activity at the hospital last night.
Further confirmed details are likely to be thin on the ground until the Royal baby is born and an official announcement is made.
The arrival will be announced in traditional fashion, with a notice on an easel behind the iron railings of Buckingham Palace.
The brief bulletin, on headed Palace notepaper, will be taken by an official from the Lindo wing of the hospital in an official car to Buckingham Palace.
Well-wishers from around the globe have been descending on the Palace since the news was announced this morning.
Royal fan Kelly arrived outside the Palace where she is waiting for news with partner Paul and their children Jack and Evie.
She told Sky News: "We've been here since 12 o'clock, we’ll probably stay another couple of hours; as long as it takes.
"We just want to share the news and the experience with everyone, it’s an historic day."
There were frantic scenes this afternoon when the Queen returned to the Palace after spending the weekend at Windsor Castle - the first real movement at the Palace for several hours.
John Jackson, head of security at the hospital, told Sky News: "We've got plans in place throughout the trust, we're very pleased with the way the plans have gone and our future Royal baby will be safe while it's here.
Larissa Milare, 25, from Sao Paulo in Brazil, said she was keen to witness the announcement in person. "It would be so special," she said. "I don't want to miss this."
Royal fans have also gathered at the hospital, where there is a strong police presence.
Among them is Terry Hutt, 78, from Cambridge, who has been sleeping outside for days and is wearing a Union flag suit and tie for the occasion.
The former soldier, who served with the Royal Ordnance Corps, said: "I have lost my voice with all the excitement.
"The health of the baby, and Kate, is the only important element."
David Cameron also wished the royal couple well. "Best wishes to them," he said. "A very exciting occasion and the whole country is excited with them. So, everyone's hoping for the best."
People from Kate's home village of Bucklebury have spoken of their excitement, with bunting out and pubs that normally shut on a Monday preparing to open when the announcement is made.
The Duchess' former music teacher, Daniel Nicholls, said: "Once the baby is born, Bucklebury will take it in its stride that the future King or Queen of England will come from here."
Kate is being tended by a top medical team led by the Queen's former gynaecologist Marcus Setchell, who delivered the Countess of Wessex's two children.
Former midwife Clare Byam-Cook told Sky News: "They'll be monitoring her very carefully to check that the baby is doing well."
Kate's due date has never been announced, but it was widely believed to be July 13.
The world's press have been outside St Mary's, in Paddington, for days in anticipation of the birth.
The impending birth has dominated the world's media from the moment the Duchess of Cambridge's labour was announced at 7.30am.
The Royal baby also accounted for seven of the top 10 trending topics on Twitter within a couple of hours of news the Duchess had gone into labour.
Others voicing their support included the Archbishop of Canterbury, the Most Rev Justin Welby, who tweeted: "My thoughts and prayers are with Kate and the whole family on this enormously special day."
Celebrities including Joan Collins, Girls Aloud singer Cheryl Cole and ex-Spice Girl Melanie Brown are among those sharing the nation's excitement.
The news that the Duchess was in labour at the Lindo wing - on what is set to be the hottest day of the year so far - was confirmed in a brief statement from Kensington Palace at 7.30am.
The Duke and his younger brother Prince Harry were born in the same wing and the Prince and Princess of Wales famously posed on the building's steps in 1982 holding baby William.
Baby deliveries at the wing start at £5,000, while consultant fees and other charges can bring the cost of a two-night stay to more than £12,000. Its other current patients include the pregnant sister of adventurer Ben Fogle, Tamara.
William is known to want a daughter, while the Duchess is hoping for a son.
Betting on the name of their first-born , which will be third-in-line to the throne, has produced one favourite with a number of bookies - Alexandra. According to Oddschecker.com, George and James are the joint top contenders for a boy.
Whatever name the couple go with, it is likely to set a trend for the next generation of infants.
Recent changes to the rules of succession mean that if it is a girl, she will not be leapfrogged to the throne by a younger brother.
The Queen will be informed of the birth in a phone call from William, according to the Queen's former press secretary Dickie Arbiter.
But he said the Royal Family will not visit because it demands an extra level of security. "The last thing they'll want is to disrupt the hospital," he said.
The birth will be an historic first with three heirs in waiting while the sovereign is fit and well, Mr Arbiter added.
Prince William will be on paternity leave from his job as an RAF search and rescue helicopter pilot for two weeks. It is not known how long Kate intends to put her Royal duties on hold so she can devote her time to the baby.
They are becoming parents more than two years after they were married at Westminster Abbey in April 2011, with the celebrations watched by millions around the world.