Fergal Roche, The Key chief executive, said: "School leaders are already struggling to retain staff and manage their teachers' workload, so add thousands more pupils arriving ill-prepared for the classroom to the equation, and the burden placed on our schools will be huge.

"An agreed definition of what 'school-readiness' means, could be the first step to helping schools, parents and early years practitioners identify what national or localised support is required to meet this growing issue."

Gareth Jenkins director of poverty policy at Save The Children said the report "provides yet further evidence that too many children are not getting the support they need to thrive in their early years".

He said: "Research for Save The Children has shown that falling behind in their early years can drastically limit a child's chances of success later on, affecting results throughout school, and even earnings as adults.”

Last year, a leading child psychiatrist suggested parents enforce a “talk not tap” rule at the dinner table to stop smartphones and tablets taking over children’s lives.

Dr Hayley Van Zwanenberg, of the Priory clinics, warned that children “transfixed” by social media and messaging risk growing up emotionally stunted and unable to cope properly with the real world.