Prince William and Kate, the Duchess of Cambridge, are expecting their third child, Kensington Palace has announced.

The Queen and the Prince of Wales are “delighted” by the news, according to a statement.

Kate, 35, is suffering Hyperemesis Gravidarum, a form of severe morning sickness, which she suffered in her previous two pregnancies.

She was hospitalised with severe morning sickness while pregnant with her first child, Prince George.

On Monday, local time, Kate was forced to cancel her planned engagement.

She was due to visit the Hornsey Road Children’s Centre in London to hear about the mental health impact of becoming a mother.

Kate is being cared for at Kensington Palace.

The statement did not say how far along Kate’s pregnancy is.

William and Kate have two children, George, four, and Charlotte, two. Their third child will be fifth in line to the British throne, bumping uncle Prince Harry out of the top five for the first time and into sixth place.

The Prince of Wales is first in line, followed by William, George and two-year-old Charlotte.

UK Prime Minister Theresa May tweeted her congratulations to the couple, calling it “fantastic news”.

The Duchess of Cambridge was last seen at a memorial garden for Princess Diana last week. Photo: Getty

Kensington Palace said in a statement: “Their Royal Highnesses the Duke and Duchess of Cambridge are very pleased to announce that the Duchess of Cambridge is expecting their third child.

“The Queen and members of both families are delighted with the news.”

The past year has already been one of big changes for the couple.

Their main home is now their Kensington Palace apartment rather than their Norfolk mansion Anmer Hall and William quit his job as a pilot for the East Anglian Air Ambulance in July to become a full-time royal.

The news was announced as the royal couple were preparing to send their eldest child, Prince George, off to school for the first time.

They have chosen a fee-paying independent school in a south London borough for their four-year-old son.

Thomas’s Battersea will welcome the young prince and his parents on Thursday.

The Duchess of Cambridge’s pregnancy follows the 20th anniversary of the death of Prince William’s mother, Princess Diana.

Kate was last seen in public visiting a memorial garden for Princess Diana last week. There was no sign of a baby bump.

During a visit to Poland in July this year, Kate was gifted clothes for a newborn and joked about having another baby.

“We will just have to have more babies,” she said at the time, sparking speculation she may have known about the pregnancy.

Prince William and Kate have two children, four-year-old George and two-year-old Charlotte. Photo: Getty

-with AAP