Kate Middleton is still extremely unwell and has pulled out of her last remaining public engagement before Christmas, a premiere of the new film The Hobbit in London tonight, as her condition continues to remain a cause for concern.

The Duchess spent three nights at the King Edward VII Hospital in London last week being treated for an acute form of pregnancy sickness called hyperemesis gravidarum, which can cause women to vomit up to 30 times a day. She was severely dehydrated and had to be put on a drip.

She was profoundly upset by the tragedy of nurse Jacintha Saldhana who committed suicide after she was fooled by a prank call to the hospital.

When she was discharged on Thursday, she told reporters she was feeling ‘much better’ but she took a turn for the worse on Sunday while she was convalescing at Kensington Palace. Such was her distress that night that William cancelled a long-planned appearance at a military event to remain by her bedside.

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

It is still hoped that William will attend the screening of the Peter Jackson film, starring Martin Freeman as Bilbo Baggins.

Kate’s attack of hyperemesis gravidarum, which can lead to constant vomiting, saw her spend three nights in hospital last week, and forced the announcement of her pregnancy.

A spokesperson for the couple said at the time: “It is well known that hyperemesis gravidarum often recurs and, until further notice, to allow the Duchess a degree of privacy during her pregnancy, we do not intend to offer regular condition checks or advise of routine developments associated with it.”

William is under pressure to get back to work as an RAF search and rescue pilot as he has to make up flying hours missed to participate in the jubilee celebrations. He will be returning to Wales tomorrow.

Kate will probably not be seen in public until Christmas Day at the earliest, when the Royal family traditionally attend church at their Sandringham Estate.