Prince William's wife Kate in labour

 Prince William's wife Kate was on Saturday admitted to hospital in London in labour with the couple's eagerly-awaited second child as royal fans camped outside celebrated the impending birth.

"The Duchess of Cambridge was admitted at 6.00 hours to St Mary's Hospital, Paddington, London and is in the early stages of labour," read a statement from Kensington Palace, their residence.

"The Duchess travelled by car from Kensington Palace to the Lindo Wing at St Mary's Hospital with The Duke of Cambridge (Prince William)," it said.

British Prime Minister David Cameron sent his best wishes in a tweet.

"The whole country will wish her well," he wrote.

The child -- a prince or princess -- will be fourth in line to the throne after Queen Elizabeth II's eldest son Prince Charles, his eldest son William, and big brother George who was born in 2013.

   'Can't wait to hear the news' 

 The world's press quickly gathered outside St Mary's in anticipation of the birth, joining a group of royal fans decked out in the Union Jack colours who have been camped outside the hospital for a week.

"It'll be a lovely toy for George!" said one of the fans, Terry Hutt, who celebrated his 80th birthday on Thursday outside the Lindo Wing, the hospital's private maternity ward where Kate has been admitted.

"I want a healthy baby. This time I think I'd go for a girl," said Hutt, who was wearing a tracksuit with a Union Jack shirt over the top and a Union Jack hat with badges of the royal family on it.

Another supporter, Kathy Martin, 52, who has remained camped out for 12 nights, said: "Words can't describe how I feel because it's been a long long time."

"I'd love to see a girl. (Prince William's mother) Diana always wanted a little girl. I think William would love to fulfil her dream for her," she said, adding: "We can't wait to hear the news!"

William was expected to stay by his wife's side throughout the labour, as he did for George's birth.

The Lindo Wing charges around £6,000 (8,110 euros, $9,083) a night for a delivery, although Kate will be getting a 10 percent discount for a second birth.

Queen Elizabeth, senior royals and Kate's parents, Michael and Carole Middleton, will be the first to be informed of the news.

It will then be announced both on the official Kensington Palace Twitter account and in the traditional way -- with a royal bulletin displayed on an ornate easel in Buckingham Palace's forecourt.

Guy Thorpe-Beeston, surgeon-gynaecologist to the royal household and a specialist in high-risk pregnancies, is leading the delivery team.

Kensington Palace had announced Kate's pregnancy in September and she struggled with severe morning sickness as she did in her first pregnancy.

Kate had said she was due in late April and William's office announced on April 21 that he had gone on leave from his job as an air ambulance pilot after completing his training ahead of the birth.

He is not expected to return to work until June 1.

Prince Charles has said he would like a granddaughter and William has told supporters the second child will be a "game-changer" after the first "life-changer".

    Gun salutes in celebration 

 William and Kate are planning to spend the first few days after the birth at Kensington Palace.

They will then travel to Anmer Hall, a secluded 10-bedroom country mansion on Queen Elizabeth's privately-owned Sandringham estate in Norfolk, eastern England, where they have set up home.

Cannons will be fired across London to celebrate the birth -- 62 shots from the Tower of London and 41 from Green Park -- and the British flag will be flown from government buildings.

The baby's name may not be revealed for several days.

William's name was not announced for a week, while the world had to wait one month after Charles was born.

George was named two days after his birth.

The baby will be known as His/Her Royal Highness Prince/Princess of Cambridge.

The baby will be christened as a member of the Church of England, wearing a replica of the intricate lace and satin gown made for queen Victoria's eldest daughter in 1841.

Experts say the royal birth could inject tens of millions of pounds into the British economy, with a baby princess particularly lucrative because she could become a fashion trendsetter.