Prince William's wife Kate was admitted to hospital on Saturday in the early stages of labour with the couple's eagerly-awaited second child, Kensington Palace said.

"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," the palace press office said in a statement.

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

This will be the couple's second child after Prince George, born in 2013.

He or she will be the fourth in line to the throne after Queen Elizabeth II's eldest son Prince Charles, his eldest son William, and George, the new arrival's big brother.

William was expected to stay by his wife's side throughout the labour, as he did at George's birth in July 2013.

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.

The world's press quickly gathered outside St Mary's in anticipation of the birth.

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.

A spokesman for Kensington Palace, William's official residence, said he would not return to work until June 1.

Gun salutes in celebration

He and Kate are planning to spend the first few days after the birth at Kensington Palace, their London residence.

They will then travel to Anmer Hall, a 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 his father Charles was born.

George was named two days after his birth.

The baby will be known as His/Her Royal Highness Prince/Princess (name) 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 (euros, dollars) into the British economy, with a baby princess particularly lucrative because she could become a fashion trendsetter.

George's birth gave Britain a £390 million (544 million euros, $592 million) boost and the new baby is expected to generate approximately £300 million, according to figures from the London School of Marketing.