British royal couple Prince William and wife Kate Middleton are expecting their third child, the Kensington Palace said in a statement on Monday (Sept 4).

"Their Royal Highnesses The Duke and Duchess of Cambridge are very pleased to announce that The Duchess of Cambridge is expecting their third child," said the statement.

"The queen and members of both families are delighted with the news."

The statement added that the Duchess is suffering from Hyperemesis Gravidarum, an extreme form of morning sickness, and is being cared for at the palace.

She also suffered from the condition in her previous two pregnancies.

The 35-year-old has two other children with Prince William: Four-year-old Prince George of Cambridge, and Princess Charlotte of Cambridge, who is two.

The duchess cancelled her planned engagement at the Hornsey Road Children's Centre in London on Monday.