Prince William visited his pregnant wife Kate in hospital on Tuesday where she was spending a second day being treated for acute morning sickness, as messages of congratulations poured in from around the world.

The announcement on Monday that the Duke and Duchess of Cambridge, both 30, are expecting their first child ended feverish speculation about a new royal heir that began immediately after their lavish wedding in April 2011.

The child will be third in line to the throne.

It will also be directly in line to become the monarch regardless of whether it is a girl or a boy, after a historic agreement among the 16 Commonwealth realms last year to end the practice of male primogeniture.

St James's Palace said Kate was still at the "very early stages" of pregnancy, she is believed to be less than 12 weeks, but it is thought the news was released because her admission to hospital would have made her condition public.

Kate has hyperemesis gravidarum, a very acute form of morning sickness which affects 3.5 in every 1,000 pregnant women.

Although it should not harm the baby if treated correctly, it can be unpleasant for the mother as it causes severe vomiting and carries a danger of dehydration.

Britain's press on Tuesday celebrated the "delightful" news that Prince William and his wife Catherine are to have a baby, although the expectant mother's bout of morning sickness tempered spirits.

Popular tabloid The Sun ran with "Kate Expectations" as its front-page headline. The Daily Telegraph said the news of Catherine's pregnancy was cause for national celebration.

"Who would not be delighted at the prospect of a mother's first child, especially a mother who has won affection with her natural beauty and straightforward character?" said its editorial.

The Telegraph's headline asked "Could it be twins for the Duchess?" pointing out that the condition is more often experienced by women expecting twins.

The Sun called the sudden announcement "fantastic news". "As William and Kate embark on this new journey, the nation wishes the nervous Royal couple well," said its editorial.

The Times splashed "We're Expecting" above its front-page story, but warned that the couple were facing a new level of press scrutiny.

"For the Duke and Duchess, the rather rushed announcement on a dull Monday afternoon represents a pivotal moment in their lives," wrote columnist Valentine Low.

Meanwhile, the former British colonies of Australia and New Zealand expressed delight Tuesday that Prince William and wife Catherine are expecting their first child, who will one day be their new head of state.

Australian Prime Minister Julia Gillard said the news "is going to bring joy to many around the world" and wished Kate Middleton a swift recovery from the morning sickness.

Gillard's New Zealand counterpart John Key said the news was "fabulous" for the young royals and would make Queen Elizabeth II and her husband Prince Philip pleased.

AFP