BRITAIN is celebrating the birth of a new royal baby, with Prince William's wife Catherine delivering a healthy baby boy overnight in London.

The precious little prince arrived at 11.01am London time (8.01pm AEST) at St Mary's Hospital in Paddington, and mother and baby were both doing well.

Prince William was by his wife's side as she went into labour earlier on Monday morning.

Britain's Prince William, Duke of Cambridge (L) stands next to his wife Catherine (R), Duchess of Cambridge who holds their newborn son outside the Lindo Wing at St. Mary's Hospital in Paddington, London, Britain, 23 April 2018. The baby boy is the royal couple's third child and fifth in line to the British throne.
Britain's Prince William, Duke of Cambridge (L) stands next to his wife Catherine (R), Duchess of Cambridge who holds their newborn son outside the Lindo Wing at St. Mary's Hospital in Paddington, London, Britain, 23 April 2018. The baby boy is the royal couple's third child and fifth in line to the British throne. EPA - ANDY RAIN

The child, whose name will be revealed in coming days, weighed 3.9kg. He is the third child for Catherine, the Duchess of Cambridge and Prince William and will be fifth in line to the throne.

"Her Royal Highness the Duchess of Cambridge was safely delivered of a son at 11.01hrs,'' Kensington Palace tweeted.

"The Duke of Cambridge was present for the birth.

"Her Royal Highness and her child are both doing well.

"The Queen, the Duke of Edinburgh (Prince Philip), the Prince of Wales (Prince Charles), the Duchess of Cornwall (Camilla), Prince Harry and members of both families have been informed and are delighted with the news,'' the palace said.

The child, who will be known as His Royal Highness Prince (name) of Cambridge, is a younger sibling to Prince George, 4, and Princess Charlotte, who is also three.

He was delivered in the private Lindo Wing, where Catherine, 36, also gave birth to her older children.

Kensington Palace had three hours earlier confirmed that Catherine had gone into labour.

The Metropolitan Police set up barricades outside the hospital as the British press and hundreds of dedicated royal fans rushed to stake out positions in the hopes of seeing Catherine and the new baby.

Royal fans who had been camped outside the hospital for hours were in tears following the news, which was announced by a town crier and burst into song and cheers.

Christine Morgan, 60, told News Corp she was delighted.

" I had inkling it would be a boy, my own daughters' (birthday) was yesterday so it's very exciting," Ms Morgan said.

"I think if they call him Albert or Bertie for short that will be just marvellous."

Sydney woman Jenny Cook lives in Paddington and said she decided to pop down to the hospital when she heard Catherine had gone into labour.

"I'm just hoping I might be able to get a glimpse (of the new baby)," she said.

"I've been living here for three years and living so close does make it more exciting."

Amy Thompson, 18, had been camping out for 15 days.

"I was just 11 when Kate and William got married," Ms Thompson said.

"I was here for George and Charlotte and just love the family."

" I couldn't be happier."

John Loughrey, 62, broke into song following the announcement.

"It's a very important day for Britain and I think St George is happy," he said, referring to one of Britain's saints.

The 36-year-old Duchess had continued working until March 22 then went on maternity leave, although she was seen attending a church service with the Queen and other royal family members at Easter.

The new baby will be fifth in line to the throne behind grandfather Prince Charles, father Prince William, and siblings George and Charlotte.

Prince Harry, William's brother, falls back to sixth in the line of succession and all other royal members shuffle down a further step.

Kensington Palace announced the pregnancy in early September after Catherine became ill with Hyperemesis Gravidarium, or extreme morning sickness. She missed several public events, including Prince George's first day at school. She had suffered the same condition in the early stages of her previous pregnancies.

The Duchess recovered after several months and resumed her busy royal schedule.

Kensington Palace said the medical staff who assisted in the birth were Mr Guy Thorpe-Beeston, Surgeon Gynaecologist to the Royal Household, Alan Farthing, Surgeon Gynaecologist to Her Majesty The Queen, Dr Sunit Godambe, Consultant Neonatologist at Imperial College NHS Trust and  Professor Huw Thomas, Physician to Her Majesty The Queen and Head of the Medical Household.

The arrival of the new baby comes at the start of a joyous few months for the British royal family.

On Saturday, Queen Elizabeth marked her 92ndbirthday by attending a speculator pop concert in London, along with most members of the royal family including Prince William and Prince Charles, raising money for charity.

On May 19, Prince Harry and his glamorous American fiancée Meghan Markle will marry at St George's Chapel at Windsor Castle, an event expected to be watched by billions of people around the globe through mainstream and social media.

Another of the Queen's grandchildren, Zara Tindall, is expecting her second child with husband, former England rugby great Mike Tindall, in coming weeks.

And Princess Eugenie, daughter of Prince Andrew the Duke of York and his former wife Sarah, the Duchess of York, will marry her fiancé Jack Brooksbank at St George's Chapel on October 12.

Bookmakers in the UK suggest the most likely names for the new baby will be Arthur or Edward.

William and Catherine had opted not to know the baby's gender in advance.

The palace will place a formal notice on an easel outside Buckingham Palace to officially announce the news.

The baby's name will be announced in coming days.

News Corp Australia

A profitable partnership

A profitable partnership

East Ballina Lions and Ballina Fair partner to support farmers

Keeping the heart beating

Keeping the heart beating

Bike ride to raise AED awareness

Skatefest going ahead

Skatefest going ahead

Fair Go Skatefest to run again this year

Local Partners