London (AFP) - Britain's Prince William said he was "thrilled" that his wife Kate is pregnant with their second child, but admitted it had been a "tricky few days" as she struggles with severe morning sickness.
Kate cancelled an official engagement on Monday because she is suffering from hyperemesis gravidarum, the same condition that caused her to be hospitalised in 2012 when she was pregnant with her first child Prince George.
William attended the public event in Oxford without her and told reporters: "It's been a tricky few days, a week or so, but obviously we're immensely thrilled, it's great news."
He said it was still "early days, but I'm hoping things will settle down and she feels a bit better", adding: "I'm going to go back to look after her now."
Kate, 32, is being treated by doctors at the couple's London home Kensington Palace, a spokesman said.
The pregnancy was announced earlier Monday in an official statement from the palace, which said that Queen Elizabeth II, William's grandmother, was "delighted" at the news.
"Their Royal Highnesses the Duke and Duchess of Cambridge are very pleased to announce that the Duchess of Cambridge is expecting their second child," the palace said.
"The Queen and members of both families are delighted with the news."
The announcement made headlines across the world, reflecting the star status of William and Kate.
The birth of their first child, George Alexander Louis, on July 22, 2013, sparked a media frenzy and prompted messages of goodwill from around the world.
On Monday, the BBC's 24-hour news station cut away from Prime Minister David Cameron's statement to parliament on last week's NATO summit to carry William's remarks live.
But the prince urged a sense of perspective.
"It's important that we all focus on the big news, and the big international and domestic things going on at the moment. That's where my thoughts are," William said.
Cameron said: "On behalf of the whole country, I am sure the House will join me in congratulating them on this fantastic news and wishing them well in the months ahead."
- 'Heir and spare' -
Prince George is third in line to the throne and his new sibling will be fourth in line, pushing William's younger brother Prince Harry down to fifth place.
Harry joked that it was "great" that he would have less of a chance of becoming king.
"I can't wait to see my brother suffer more," Harry said, adding that he would love to see William "try and cope" with a girl.
"I think George will be over the moon. I think he will be thrilled having another small younger brother or sister," the prince said.
William and Kate made no secret of wanting another child, and it is the convention in aristocratic and royal families to have at least two -- an heir and a "spare" who can inherit the family title and estate in case the heir dies.
"I hope it's a girl," one woman wellwisher told AFP outside Kensington Palace. "An heir and a spare! So he's sorted, isn't he?"
British bookmakers said the announcement had caused a flood of bets to be placed on the baby's name and gender.
Coral said 70 percent of bets were on the baby being a boy, and that James, Alexander and Louis were the favourite names.
For a girl, Elizabeth, Diana and Victoria were the favoured names.
Bookmaker Ladbrokes was offering odds of 20 to 1 for Kate to have twins.
- Severe sickness -
The extent of Kate's morning sickness raises questions about whether she will be able to fulfil her first official solo trip abroad to Malta, scheduled for September 20-21.
She was due to represent the queen as the Mediterranean islands celebrate the 50th anniversary of their independence from Britain.
A Kensington Palace spokesman said the duchess's plans would be decided on a "case by case" basis.
Kate has been increasing her public profile since she married William in a lavish ceremony in April 2011.
A "commoner" by birth who met the prince at university in Scotland, she appears to have moved effortlessly into her royal role and has become hugely popular with the public, as well as a global style icon.
But the couple have worked hard to keep their family life private after the press interest in William's mother Diana, who was killed in a car crash in Paris in 1997 as her car was chased by paparazzi.
Kensington Palace did not say when the new baby was due, although experts say hyperemesis gravidarum usually occurs in the first three months of pregnancy.
It affects around one in 200 pregnant women, according to Britain's National Health Service (NHS), and symptoms can include persistent vomiting, tiredness and dizziness.
Hospitalisation may be needed in severe cases to treat dehydration with intravenous fluids for a few days, and Kate was admitted for three nights during her last pregnancy.
The couple currently live in London but William, a helicopter pilot, is due to begin a full-time role next year with an air ambulance charity based in Cambridge, eastern England.