But the source said today: "It's looking highly unlikely now. The Duchess wanted more than anything to be there. It's a big day for her son. But she has been really very sick this week."
Kate was forced to pull out of two public engagements on Monday and Tuesday after suffering from hyperemesis gravidarum, as she did in her previous pregnancies.
On Tuesday - a day after it was announced they are expecting their third child - William said Kate was well, but admitted: "There's not much sleep going on at the moment."
He said: "We need Catherine to get over this first bit and then we can start celebrating.
"It's always a bit anxious to start with, but she's well."
Prince George and his parents are due to be greeted by Helen Haslem, head of lower school, on their arrival at the school gates. They will then be escorted to his reception classroom.
The young prince will be dressed in a smart new uniform, comprising a navy v-neck pullover, matching Bermuda-style shorts, long red socks and black shoes.
His new school, where fees cost from £17,604 a year, has been described by the Good Schools Guide as: "A big, busy, slightly chaotic school for cosmopolitan parents who want their children to have the best English education money can buy.
"That is what they want and, to a large degree, that is what they get."
George and his classmates will be taught a range of subjects and activities from ballet and art to drama, French, music and physical education.
Kate has already said she is not sure her son "has any idea what's going to hit him" when he starts school.
What is Hyperemesis Gravidarum?
Hyperemesis Gravidarum (HG), which strikes just one per cent of pregnancies, is a complication that causes excessive nausea and vomiting.
Unlike regular morning sickness, it doesn't fade away with time and can leave some women bedbound as they are unable to keep food or drink down.
It is also considered to be the second leading cause of hospitalisation during pregnancy and can lead to dehydration - dangerous to both the mother and child.
If dehydration does strike, babies are at risk of deformities because the constant vomiting can deprive the woman's body of amniotic fluid - which the baby needs to thrive.
The condition may not clear up completely until the baby is born, although some symptoms may improve at around 20 weeks.
Some pregnant women are sick many times a day, which can have a negative effect on their daily life.
Many mothers-to-be may lose up to 10 per cent of their body weight when they are supposed to be gaining about 1lb a week.
This can trigger a build-up of toxins in the blood or urine known as ketosis as the body tries to compensate for lack of food.
Exactly how many pregnant women get HG is not known as some cases may go unreported, but it's thought to be around one in every 100.
