Photographers are camped outside the hospital, social media is abuzz and stores are touting baby goods ahead of the expected arrival this week of the future heir to the British throne.

Britain is officially on baby watch, with Prince William and his wife, Catherine, awaiting the arrival of their first child, who will be third in line to the throne.

The couple - known as the Duke and Duchess of Cambridge since their sumptuous royal wedding in April 2011 - announced last December that a baby was on its way, after the Duchess was admitted to hospital for four days due to severe morning sickness.

With the due date looming, the Duchess stopped official duties last month while Prince William, a search-and-rescue helicopter pilot and grandson of Queen Elizabeth II, is on standby at an air-force base in North Wales to rush back to London.

The couple have remained vague about the exact date the baby is due, other than to say the middle of this month. The arrival will be announced through a combination of the traditional and modern: via Twitter, websites and a notice outside Buckingham Palace.

In a sign that the baby's arrival was getting closer, royal officials announced that the child would be known by its given name and would have the title His or Her Royal Highness Prince/Princess of Cambridge.

Mr Joe Little, managing editor of Majesty Magazine, said the Duke and Duchess were a private couple and, while aware of the massive global interest, were trying to limit public exposure.

"Privacy is key for Prince William as he saw the way his mother, Princess Diana, suffered at the hands of the paparazzi, and he wants to make sure this does not happen to his wife or his own children," Mr Little told Reuters.

The baby is to be born in the private Lindo wing of St Mary's Hospital in London, where Prince William was born 31 years ago. Princess Diana, who died in a car crash in 1997 after splitting up with Prince Charles, also had Prince Harry there.

The gender of the baby remains unknown.

The couple said they do not know the gender, despite wide speculation in March that it was a girl after the Duchess, 31, accepted a baby gift saying: "Thank you, I will take that for my d..."