Prince William pulled out of a public appearance in London on Sunday (09Dec12) as his pregnant wife Catherine, Duchess Of Cambridge struggled through another serious bout of morning sickness.

The couple announced its baby news last week (03Dec12) as the Duchess checked into the private King Edward VII Hospital in London to receive treatment for the debilitating condition hyperemesis gravidarum.

She was discharged on Thursday (06Dec12) and has been continuing her recovery at their royal residence Kensington Palace, but Catherine fell ill again this weekend (08-09Dec12), prompting Prince William to cancel plans to appear at the British Military Tournament to be by his partner's side.

A royal spokesman tells UsMagazine.com, "It is well known that hyperemesis gravidarum often recurs... 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."

Meanwhile, Prince William, who is second in line for the throne, managed to make light of his wife's illness as he attended the Winter Whites Gala in London on Saturday night (08Dec12).

He quipped, "I don't know why they call it morning sickness - they should call it all day and all night sickness", adding, "(Recovery is) a long old process but she is getting there. She feels like it is going to go on forever."

The parents-to-be wed last year (11).