Prince George has walked nervously into school on his first day with his father by his side but without his mother, the Duchess of Cambridge.
Kate had hoped to join the Duke of Cambridge on the school run but is suffering from severe morning sickness and was too ill to attend, Kensington Palace said on Thursday morning.
The four-year-old prince was helped out of the car by William after being driven through the gates of Thomas's Battersea, a fee-paying independent school in south London.
The prince would be known as George Cambridge to his classmates, the palace said.
George looked tentative, clutching his father's hand, as he was led towards the school entrance.
The young prince shook hands with Helen Haslem, head of the lower school, as the Duke held his school bag ready for his first day.
Then, with William holding one hand and Haslem the other, he walked inside.
It is understood William and Kate will try to do the school run as often as possible.
The prince, who was wearing a shirt, navy V-neck pullover, shorts and black shoes on his first day, is thought to be one of the youngest in his year.
Along with his classmates, he will spend the day getting to know teachers, adults and other children as well as completing the important task of finding his classroom peg.
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."
George's new school, where fees cost from STG17,604 ($A28,738) 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."