The pregnant Duchess of Cambridge had been due to take on her first royal overseas solo engagement this weekend, visiting Malta to celebrate the 50th anniversary of the island's independence.
But, after consulting with the royal's doctors, Kensington Palace has now announced that Kate, 32, is unable to travel due to her condition.
Her husband, Prince William, will take her place.
"The Duke of Cambridge is honoured to represent The Queen on the visit, which commemorates the 50th anniversary of Malta's Independence," said the palace.
As happened during her first pregnancy, Kate is once again battling hyperemesis gravidarum, a very severe form of sickness. It led to her being briefly hospitalised during her first pregnancy with Prince George.
A statement released on 8 September said: "Their Royal Highnesses The Duke and Duchess of Cambridge are very pleased to announce that The Duchess of Cambridge is expecting their second child. The Queen and members of both families are delighted with the news.
"As with her first pregnancy, The Duchess is suffering from hyperemesis gravidarum. The Duchess of Cambridge is being treated by doctors at Kensington Palace."
Kate's illness means she's already pulled out of a string of planned public appearances in recent weeks including the Invictus Games Opening Ceremony and a joint engagement with Prince William in Oxford.
Speaking to reporters in Oxford shortly after Kate's pregnancy was publicly announced, her husband William, 32, said: "She's feeling okay, thanks. It's been a tricky few days, week or so, but we're immensely thrilled. It's great news. Early days. Hopefully things settle down and she feels a bit better."
Kate and William welcomed their first child together, Prince George, in July 2013.
That first pregnancy was announced in December 2012 after Kate was admitted to hospital in London with hyperemesis gravidarum. She remained at King Edward VII Hospital for three days before being released.
Stars congratulate the Duke and Duchess on their baby news: