Kate Middleton, who is three months pregnant with her second child, had to pull out of attending the launch of charity The Art Room's latest project due to her pregnancy-related sickness.
The Duchess has been suffering from hyperemesis gravidarum for several weeks, and had already cancelled what would have been her first solo foreign tour, to Malta, last month.
Her spokesman said: "The Duchess of Cambridge continues to suffer from the effects of hyperemesis gravidarum and was unable to attend this event today in person.
"The decision to undertake engagements is being kept under review on a case by case basis."
Middleton penned a message to be read out at the launch of the Clore Art Room at Barlby Primary School, London.
She wrote: "I wanted to share my congratulations with you all for the launch of the new Clore Art Room. I was looking forward to joining you all for this particularly special day as part of The Clore Duffield Foundation's 50th Anniversary year.
"I am truly sorry that I cannot be with you all today as you celebrate this milestone. As patron of The Art Room I feel great pride to see the work that the charity is doing.
"Vulnerable children flourish in the safe havens that The Art Room provides; this highlights just how important it is to support children at a time when they most need help and support."
She added: "I hope that you have a wonderful afternoon. My sincere apologies for not being able to join you but I hope that I will have an opportunity to visit Barlby and see the Clore Art Room soon."
Kate has been patron of The Art Room since 2012, which aims to provide emotional support for children by encouraging them to express themselves through art.