Prince George is not going to be an only child for long — the toddler will soon have a baby sister or brother to share his fancy digs at Kensington Palace.

British royal officials said Monday that Prince William and the duchess of Cambridge, the former Kate Middleton, are expecting their second child.

For the second time, Kate is being treated for acute morning sickness in the early phases of her pregnancy.

The first time she was so ill she required hospitalization. This time, she is being treated by doctors at her residence in Kensington Palace. She cancelled a planned engagement in Oxford to rest and receive medical care.

Prince William told well-wishers in Oxford that Kate should be over the worst of her symptoms in a few weeks.

Britain Royal Baby

Television news satellite trucks line up to report on the second pregnancy of Britain's Kate, Duchess of Cambridge, and Prince William outside Kensington Palace in London. (The Associated Press)

"She wishes she could be here," he said.

"She's feeling OK, thanks. It's been a tricky few days — week or so — but obviously we're basically thrilled, it's great news, and early days. We're hoping things settle down and she feels a bit better."

The new baby, boy or girl, will become fourth in line to the throne, pushing Prince Harry to fifth. George, who is 13 months old, is third and likely to become Britain's monarch one day. William is second in line, while his father, Prince Charles, is first.

Britain had changed its laws before George's birth so that the couple's first born would be in line for the throne regardless of its sex. Before the change, a girl would have lost her place in line if a boy was born later.

William and Kate have often expressed an interest in having a larger family.

Royal couple 'delighted'

The royal couple and their families are "delighted" with the baby news, said officials at Clarence House, the couple's office. The announcement follows months of speculation in the glossy British and American press about a possible baby brother or sister for George.

Prince Harry said he's excited at the prospect of becoming an uncle again. William's younger brother

"I can't wait to see my brother suffer more, and with any luck, if it's a girl, he'll suffer even greater," joked William's younger brother, who turns 30 on Sept. 15.

As for his sister-in-law, Harry said her morning sickness "is a pretty horrible thing to have to go through."

Kate recovered and gave birth to George in July 2013 without further complications after hospital treatment for severe morning sickness, a condition called hyperemesis gravidarum,

The current illness means the 32-year-old duchess may need extra hydration, medication and nutrients.

Britain's Press Association reported that Kate's pregnancy hasn't passed the 12-week stage, which is when she became ill in her first pregnancy.

British Prime Minister David Cameron said he was "delighted by the happy news that they're expecting another baby."

Royal officials said it wasn't clear if the duchess will be able to carry out planned official engagements, including a trip to Malta on Sept. 20 and 21 that would mark her first overseas solo trip.

Decisions on events would be made on a "case-by-case" basis, officials said.