Prince George has started his first day at school - but his mother, the Duchess of Cambridge, missed the occasion as she was not well enough to take him.
The Duke of Cambridge dropped him off as Kensington Palace said Catherine, who is suffering from severe morning sickness, was still unwell.
The four-year-old is attending Thomas's Battersea, an £18,000-a-year preparatory school in London.
His uniform includes navy shorts and jacket, long red socks and black shoes.
The milestone for the prince comes after the duke and duchess this week announced they are expecting their third child.
Catherine had to pull out of public engagements on Monday and Tuesday because she is suffering from hypremesis gravidarum, as with her previous two pregnancies.
Prince William drove his son through the school gates before helping him out of the car.
The duke carried Prince George's bag and held his hand as the pair walked up to the entrance.
Father and son were then greeted by Helen Haslem, head of lower school, and the young prince shook her hand before being accompanied to his first class.
Prince William told Ms Haslem that George had been eagerly watching and he had "noticed" the other boys going into the school in their uniforms.
What is Prince George's uniform?
The uniform for Thomas's Battersea is only available at the Peter Jones department store in Sloane Square or from the second-hand uniform shop at the school.
For boys in reception class, the outfit consists of a navy jacket, a v-neck jersey and navy Bermuda shorts.
In winter, this is accompanied by a red polo neck and long red and navy socks.
In summer, short-sleeved light blue shirts are required - which Prince George was sporting on his first day - along with navy ankle socks.
George has joined a class of 20 other pupils and has his own desk and peg with his name on. He will begin by doing half days as a "gentle" introduction to school life.
Ben Thomas, principal of Thomas's London Day Schools, said there would be no special treatment for the royal.
"We are going to try and make this a really happy, safe and secure time," he said.
"I hope he will have the confidence to be himself with all his quirks and his idiosyncrasies and characteristics - that's what I would want for all of our children."
Mr Thomas added: "The global spotlight has been a little bit daunting but that will move on."
Notable alumni at Thomas's Battersea include pop singer Florence Welch - from Florence and the Machine - model and actress Cara Delevingne and Fresh Meat star Charlotte Ritchie.
The school was 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."
One cameraman and one photographer were at the school to capture Prince George's first day, as Prince William has previously pleaded for the press to respect his son's privacy.
Royal commentator Richard Fitzwilliams said: "William is determined to give his family as much privacy as possible and have a life as 'normal' as possible but this is extremely difficult.
"The royals have made several complaints to IPSO [the Independent Press Standards Organisation].
"They are in the royal 'goldfish bowl' and will continue to be the centre of national and international fascination."
Princes William and Harry went to Wetherby School in Kensington, before both going on to Eton College, in Berkshire.
Their father, Prince Charles, was initially educated at home by a governess, but later attended the Cheam Preparatory School, in Berkshire.
No media frenzy as royals plead for privacy
By Peter Hunt, royal correspondent
When Prince George enters the school building, at the start of his first day there, the moment will be captured by one photographer and one camera crew.
When his father made a similar journey, to a different school, three decades ago, many more members of the media were present to record a fresh developmental stage in the life of a future king.
Prince William remembers and doesn't want history to repeat itself.
George's schooling will be the latest test of the Cambridges' wary relationship with the press. They expect reporters to respect the privacy of their son and his classmates.