Britain's Prince William & his wife Kate are right now having their third child, their office said on Monday after the Duchess was forced to cancel an engagement due to severe morning sickness.

The baby will be fifth in line to the British throne, after grandfather Prince Charles, father William & elder siblings George, 4, & Charlotte, 2.

Britain's monarchy has ridden a wave of public support in recent years because of the emergence of the younger royals, William, Kate & William's brother Harry.

Kate last appeared with the princes on Wednesday when they visited a public garden at Kensington Palace dedicated to Diana but the palace said she would not attend an engagement at a children's center on Monday due to ill health.

"As with her previous two pregnancies, The Duchess is suffering from hyperemesis gravidarum.," the palace said.

"The Duchess is being cared for at Kensington Palace."

The condition led to Kate's hospitalization in the early months of pregnancy with her first child.