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".
Kate Middleton is suffering from the condition known as hyperemesis gravidarum which leaves expectant mothers with constant nausea and regular vomiting.
Prince William will still attend the screening of the latest blockbuster from director Peter Jackson, who created the popular "Lord Of The Rings" movie trilogy, The Sun reported.
"The Duke of Cambridge will attend as planned," the spokesman said.
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.
Caitlin Dean, 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.
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.
Kate spent three days being treated at London's private King Edward VII's Hospital last week and was likely to have received fluids and anti-sickness medicine intravenously.
