LONDON — 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 canceled 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.

“She wishes she could be here,” he said.

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.