The Duke of Cambridge is expected to return to his pregnant wife's bedside as she continues her hospital treatment for severe morning sickness.

William spent Monday with Kate after the royal couple announced the Duchess was expecting her first child but had developed a rare condition which can leave expectant mothers dehydrated and under weight.

Known as hyperemesis gravidarum it can leave women unable to keep down food and fluids, and carrying twins may increase the chances of developing the illness.

The world's media are camped outside the private King Edward VII Hospital in central London where the Duchess is expected to remain a patient for the next few days.

William left her bedside on Monday night without commenting to the waiting press and returned to his apartment at Kensington Palace, but like any other concerned husband he could be making the return trip later. Visits from the Duchess' immediate family - her parents Carole and Michael Middleton and sister Pippa - are also likely.

But while there is concern for the royal couple, there is excitement across the UK and beyond with messages of support sent from leading figures both at home and abroad.

Prime Minister David Cameron led the congratulations from the nation on Monday when he said: "It's absolutely wonderful news and I'm delighted for them. I'm sure they will make absolutely brilliant parents, and I'm sure everyone around the country will be celebrating with them tonight."

The Queen and Duke of Edinburgh, the Prince of Wales and members of both families are delighted with the news, said a St James's Palace spokesman.

Congratulations for the Duke and Duchess poured in from around the world. Julia Gillard, prime minister of Australia, called it "delightful news" and said it would "bring joy to many around the world", while John Key, prime minister of New Zealand, said the nation's women would sympathise with Kate's condition and wished her a speedy recovery.

Kate's illness can also lead to the build-up of toxins in the blood and urine. The Duchess is likely to be taking anti-sickness tablets and have a drip in her arm so she can receive fluids intravenously. But her symptoms of severe vomiting may last for much of her pregnancy.