The Duchess was hospitalised last week (begs03Dec12) suffering from hyperemesis gravidarum, which leaves expectant mothers feeling severely sick in the early months of pregnancy.
She was later released to recover at Kensington Palace, the London residence she shares with her husband Prince William, but is still not well enough to walk the red carpet at the U.K. premiere of the highly-anticipated Lord of the Rings prequel.
A spokesperson for St James's Palace says, ""The Duchess of Cambridge will not attend The Hobbit premiere tomorrow evening and will continue to rest privately. The Duke of Cambridge will attend as planned.""
The early stages of the royal pregnancy have been blighted by Catherine's health and an ill-fated stunt by two Australian DJs, who called London's King Edward VII Hospital pretending to be members of Britain's royal family and gained access to the Duchess' private medical information.
The nurse who took the call, Jacintha Saldanha, was found dead three days later of a suspected suicide, sparking an international backlash against radio hosts Mel Greig and Michael Christian. Their show has since been taken off air.