The Duchess of Cambridge was due to attend the royal screening of Peter Jackson's new film in London tomorrow night (December 12), but will remain at home due to her ongoing illness.
Middleton had previously been admitted to London's King Edward VII Hospital last week suffering from hyperemesis gravidarum - an acute form of morning sickness - prompting the early announcement of her pregnancy.
She was discharged from the hospital last Thursday, but her condition worsened on Sunday (December 9) while she was recovering at Kensington Palace.
A spokesman for St James's Palace said: "The Duchess of Cambridge will not attend The Hobbit Premiere tomorrow evening and will continue to rest privately."
Prince William still plans to attend the film premiere in London however, according to the Daily Telegraph.
The first instalment of The Hobbit trilogy opens this Thursday (December 13) in the UK and debuts on Friday (December 14) in the US.
Watch a TV spot for The Hobbit: An Unexpected Journey below: