Helen and Nick Cook’s first child was 4½ when he started full-time school. “There was a big difference between him and his five-year-old friends in terms of academic level,” Mrs Cook said.

William, who is now 8 and attends Thomas’s London Day School in Clapham, South London, was confident starting school but his mother found it difficult to see him disappear among the bigger boys.

“William was young mentally and with your first child it’s always more difficult,” Mrs Cook said. “He was learning to recognise letters but a number of classmates were able to string letters together and almost able to read.”

Children in England start school in the academic year in which they turn 5. “At that age, six months is a huge difference between children, and the youngest ones are playing catch-up really,” Mrs Cook, 39, said.

William’s younger brother Oliver, 6, who was also among the youngest in his class, was still used to taking a mid-morning nap when he started going to school for the whole day. “That had to stop when he went to school, so he was quite tired by the end of the day,” his mother said.

The Government’s targets for early years education are that all 5-year-olds should be able to write in short sentences and use punctuation, leaving little time for play in the first years of school.

The Cooks’ youngest child, Jamie, who is 4½, was eager to start school this term.

“He has gone in and is getting on with it. He is very happy and there is no clinginess,” Mrs Cook said.

Jamie is keen to learn his letters and wants to know how to read. His parents think that he needed the stimulation of being at school because as the third sibling he developed quickly.

“I wouldn’t want to keep him at home until he is 6 because he would be missing out on two or three years of education,” she said.

“They are like sponges at that age and as long as they are happy in their environment, there is no reason not to send them.”

Times recommends

• Pupils to be given lessons on debt
• Children 'reaching 3 without saying a word'
• Tories want children in care at boarding school