Princess Charlotte accompanied by her father and mother arrives for her first day of school at Thomas's Battersea in London on Thursday. AFP

Four-year-old Charlotte, dressed in the school's uniform of navy blue with red trim cardigan and blue skirt, held her mother's hand as she arrived at the school, accompanied also by her father and her brother.

Strict about children's appearance, the London school emphasises that hair longer than shoulder length must be tied back. Charlotte had done so neatly, with a whispy ponytail. She also bore a backpack with a sequin-studded unicorn keychain.

Helen Haslem greets Princess Charlotte at Thomas's Battersea in London on Thursday. Reuters 

Headteacher Helen Haslem greeted Charlotte with a handshake as the princess balanced on tip-toe to introduce herself, watched by a beaming Kate, who wore a modest floral dress.

The great-granddaughter of Queen Elizabeth, Charlotte is fourth-in-line to the British throne, after — in ascending order — six-year-old George, her father William and her grandfather Prince Charles.

Charlotte's younger brother, Louis, who is fifth-in-line to the throne, celebrated his first birthday in April.

George has been a pupil at Thomas's Battersea since 2017. He became the focus of attention last month when US.television host Lara Spencer commented on reports he was taking ballet lessons by saying "We'll see how long that lasts." She issued a hasty apology after being criticised for insensitivity.

Princess Charlotte is welcomed by Helen Haslem on her arrival for her first day of school at Thomas's Battersea. AFP

The school teaches 560 boys and girls between the ages of 4 and 13, with yearly fees for four year-olds of around £19,000 ($24,000).

The charge for Charlotte, however, will be discounted because her brother is already a pupil. The termly fee for a second child is £6,305 against £6,429 for the first, according to the school's website.

Two years ago police reviewed security arrangements at the school after a 40-year-old woman was arrested on suspicion of attempted burglary there shortly after George started.

The woman accepted a caution for causing a nuisance on school property.

Reuters