Britain's Prince William and his wife Catherine do not know the sex of their baby, which is expected in mid-July and should be born by natural delivery, palace sources said Wednesday.

The baby, which will be third in line to the throne, will be born in the same London hospital where princess Diana gave birth to William and his younger brother Harry.

While the monarchy is on Twitter, the birth will be announced to the world in the traditional method -- a notice posted on an easel in Buckingham Palace forecourt.

William, the Duke of Cambridge, will take paternity leave from his role as a Royal Air Force search and rescue pilot, though it is not known how long the duchess will take off royal duties.

William is expected to be at his wife's bedside for the birth.

The baby will be born in the private Lindo wing of St Mary's Hospital in Paddington, central London, where William was born in 1982 and his brother Harry in 1984.

The updates from palace sources are the first confirmed details about the birth, expected within weeks.

"Whilst this is a deeply personal and private moment for the duke and duchess, they recognise, of course, that it is also a moment of national celebration," a St James's Palace spokesman said.

William and Kate want the sex of their baby to be a surprise when it arrives.

"The duke and duchess do not know the sex of their baby and they've decided not to find out beforehand," a source said.

Kate said on a public engagement in March that she did not know whether she was carrying a boy or a girl.

If the baby is a girl, it could not be overtaken in line to the throne by any future younger brothers, following a change in the succession laws hastily agreed by the 16 Commonwealth realms, which include Britain, Canada, Australia and New Zealand.

The new baby will be a prince or a princess and will enjoy the title of royal highness, following a ruling by Queen Elizabeth.

The St James's Palace spokesman appealed for "sensitivity, dignity and privacy", given the events surrounding Kate's stay in hospital in early December, when her pregnancy was announced.

An Indian-born nurse at the King Edward VII's Hospital in London was found hanged several days after putting through a prank call from two Australian radio presenters, which resulted in details of Kate's acute morning sickness being made public.

"We appreciate that there will be mass interest in the duke and duchess and their baby over the course of the next few months and many people will want to share in their happiness, most significantly around the time of the birth," the spokesman said.

"With the events of King Edward VII's Hospital still strong in our memories, we would expect any media covering the Duchess of Cambridge's hospitalisation to ensure that the normal functions of the hospital are not impeded by any media presence."

The couple could spend the weeks following the birth at Amner Hall, a large property on Queen Elizabeth's private Sandringham estate in Norfolk, eastern England. The royals traditionally spend their Christmas break there.