Prince William and Kate's baby due in July: Palace

"Their Royal Highnesses the Duke and Duchess of Cambridge are delighted to confirm they are expecting a baby in July," a St James's Palace spokesman said.

"The Duchess's condition continues to improve since her stay in hospital last month," he added.

The update is also further confirmation that Kate is expecting just one baby after speculation that she might be carrying twins, the British media reported.

The news of the Duchess' pregnancy was released on December 3, after months of speculation.

The 31-year-old Duchess is believed to be around 13 to 14 weeks pregnant and confirmation of the birth month means it is likely she has now had her 12-week scan.

As part of a royal decree issued by Queen Elizabeth II, the baby will take the title of either prince or princess and be addressed as His or Her Royal Highness.

The Queen issued a Letters Patent, which removes an anomaly that has been in place since 1917.

The previous royal decree by George V stipulated that a son would become a prince, but a daughter would not become a princess.

If a new law published last month, the Succession to the Crown Bill, is cleared by Parliament, the couple's first child will become monarch, whether it is a boy or girl.

It came after consent was given by all 15 Commonwealth realms to press ahead with the legislation and will soon be voted on in the House of Commons.

The new legislation will end the system of male primogeniture, allowing male heirs precedence over women in line to the throne.

The bill will also end the ban on anyone in the line of succession marrying a Roman Catholic.

The Duchess was less than eight weeks pregnant and staying with her parents, Michael and Carole Middleton, at their 4.85 million pound country mansion in Berkshire when she fell ill at the beginning of December.

As her condition worsened and she began to become severely dehydrated, the decision was made to admit her to the King Edward VII Hospital in London for treatment on December 3.

Doctors diagnosed her with hyperemesis gravidarum, an acute form of pregnancy-related sickness.

Later that day the palace reluctantly announced details of her pregnancy, stressing that she was well under the crucial 12-week stage at which such pronouncements are normally made.

If the new prince or princess arrives early, he or she could share a birthday with William's mother, the late Diana, Princess of Wales, born on July 1. The due date looks likely to be around mid-July.