The Duchess of Cambridge's pregnancy sickness will almost certainly rule her out of attending the UK premiere of The Hobbit, a charity said.
Kate and William are guests of honour at Wednesday's screening of the latest blockbuster from director Peter Jackson, who created the popular Lord Of The Rings film trilogy.
But it appears over the weekend the Duchess's bouts of vomiting intensified to such a degree the Duke pulled out of the British Military Tournament on Sunday to spend more time with his wife.
St James's Palace said a decision on whether Kate would join the movie audience at the Odeon, Leicester Square would be made nearer the time.
A spokeswoman for Pregnancy Sickness Support, a charity which helps women with Kate's condition known as hyperemesis gravidarum, said the Duchess would be experiencing constant nausea with regular vomiting.
Caitlin Dean, who suffered from the same illness during three pregnancies and is a trustee of the charity, said: "I would be amazed if she's able to go to the premiere. When she's more like 14-plus weeks possibly she will have days when she can do that."
Mrs Dean added: "The sickness is constant and ongoing, it's just a matter of keeping on top of it. She might have days that are a bit better than other days but realistically you don't have days when you're actually well."
Kate, who is thought to be around nine weeks pregnant, spent three days being treated at London's private King Edward VII's Hospital and was likely to have received fluids and anti-sickness medicine intravenously.
But Mrs Dean said the Duchess's journey home to Kensington Palace could have triggered nausea and sickness. She said: "Even a car journey could have set off that downward spiral and by the time you get home you're on the way to being dehydrated."
William's decision to cancel an appearance at a public engagement to remain with Kate was welcomed by the trustee. Mrs Dean said: "Having support from her partner makes the whole thing more bearable."