The most widely anticipated pregnancy since Princess Diana's in 1981 is official: Prince William's wife, Kate, is pregnant.

St. James's Palace announced the pregnancy on Monday, saying that the Duchess of Cambridge - formerly known as Kate Middleton - has a severe form of morning sickness and is currently in a London hospital.

William was at his wife's side for most of the evening, before leaving the hospital shortly before 8:30pm.

News of the pregnancy drew congratulations from across the world, with the hashtag "royalbaby" trending globally on Twitter.

The couple's first child will be third in line to take the throne - leapfrogging Prince Harry and possibly setting up the first scenario in which a UK female heir could benefit from new gender rules about succession.

The palace would not say how far along the 30-year-old duchess is, only that she has not yet reached the 12-week mark.

Palace officials said the duchess was hospitalised with hyperemesis gravidarum, a potentially dangerous type of morning sickness where vomiting is so severe no food or liquid can be kept down.

They said she was expected to remain hospitalised for several days and would require a period of rest afterward.

The condition is thought to affect about one in 200 pregnant women, according to Britain's health department.

William is second in line to the throne after his father, Prince Charles, so the couple's first child would normally become a monarch - eventually.

The confirmation of Kate's pregnancy caps a jam-packed year of highs and lows for the young royals, who were married in a lavish ceremony at Westminster Abbey last year.

They have travelled the world extensively as part of Queen Elizabeth II's Diamond Jubilee celebrations and weathered the embarrassment of a nude photos scandal, after a tabloid published topless images of the duchess.