Britain's Prince William leaves the King Edward VII hospital in London, December 3, 2012. Britain's Prince William and his wife Catherine, Duchess of Cambridge are expecting a baby, the prince's office said on Monday. The statement said Catherine was in the King Edward VII Hospital in Central London suffering from very acute morning sickness. [Photo/Agencies]

BABY WILL BE KING, OR QUEEN

William, Queen Elizabeth's 30-year-old grandson, is second in line to the British throne, and their first child will become the third in succession when he or she is born.

Last year Britain and other Commonwealth countries which have the queen as their monarch agreed to change the rules of royal succession so that males would no longer have precedence as heir, regardless of age.

The agreement also means an end to a ban on a future monarch marrying a Catholic, a stipulation dating back some 300 years.

Britain's royal family are currently riding the crest of popularity on the back of William and Kate's wedding and the queen's diamond jubilee this year which has witnessed nationwide celebrations.

"It's something everyone can look forward to, just like their wedding brought the whole nation together," said Johanna Castle, 25, a sales assistant in an east London homewear and fashion store.

The young royal couple have become global stars after some two billion people tuned in to watch their glittering marriage ceremony and the sumptuous display of pageantry that accompanied it, and barely a day goes by without a picture of Catherine appearing in the pages of Britain's royalty-obsessed newspapers.

The duchess, the first "commoner" to marry a prince in close proximity to the throne in more than 350 years, is now a fashion icon, with her attire scrutinized every time she steps out in public and followed by legions of women around the world.

Related photos: Duchess of Cambridge a fashion icon

US President Barack Obama and his wife Michelle were one of the first to send congratulations, an indication of the young royals' popularity across the Atlantic.

"I know they both feel that having a child is one of the most wonderful parts of their lives. So I'm sure that will be the same for the Duke and Duchess of Cambridge," said White House spokesman Jay Carney.

With their fame has come unwanted attention, and there was anger in Britain when topless photos of Kate relaxing on holiday were published in a French magazine in September.

Related: UK royal win injunction on topless photos

The pictures rekindled memories of the media pursuit of William's mother, Princess Diana, who was killed in a car crash in Paris in 1997 while being chased by paparazzi.

"I will be very surprised if this isn't handled with the utmost tact and sensitivity," said media commentator Steve Hewlett. "Newspapers realize there's a huge amount of goodwill towards Will and Kate, and they take their cue from their readers."