The Duchess of Cambridge's pregnancy sickness has forced her to cancel an appearance at Wednesday's UK premiere of The Hobbit.

Kate is suffering from the condition known as hyperemesis gravidarum which leaves expectant mothers with constant nausea and regular vomiting.

A St James's Palace spokesman said: "The Duchess of Cambridge will not attend The Hobbit premiere tomorrow evening and will continue to rest privately."

William will still attend the screening of the latest blockbuster from director Peter Jackson, who created the popular Lord Of The Rings movie trilogy.

Over the weekend, the Duchess's bouts of vomiting intensified to such a degree that the Duke pulled out of the British Military Tournament on Sunday evening to be with his wife.

St James's Palace has stressed it will not be giving daily updates about Kate's health.

But although she is still suffering from the condition she may be experiencing less vomiting, so the Duke feels able to attend the premiere.

A spokeswoman for Pregnancy Sickness Support, a charity which helps women with hyperemesis gravidarum, said the royal would be experiencing constant nausea with regular vomiting.

Caitlin Dean said it would be a few months at least before Kate, who is thought to be nine weeks pregnant, could think about attending a public event.

Mrs Dean, who suffered from the same illness during three pregnancies and is a trustee of the charity, said: "When she's more like 14-plus weeks possibly she will have days when she can do that."