Vocabulary scores of more than 12,000 children showed that the sons and daughters of graduates were 10 months ahead of those with the least-educated parents, the research discovered.

A second "school readiness" assessment measuring understanding of colours, letters, numbers, sizes and shapes that was given to more than 11,500 three-year-olds found an even wider gap - 12 months - between the two groups.

The equivalent gaps for children in families living above and below the poverty line used by the researchers were five months for vocabulary and 10 months for school readiness.

Girls did better than boys on average. They were three months ahead on both measures.

Scots children were three months ahead of the UK average in their language development and two months ahead in "school readiness".

The assessments were conducted on behalf of the Centre for Longitudinal Studies, which is based at the Institute of Education, University of London.

They form part of the Millennium Cohort Study (MCS), which is tracking more than 15,500 children born in 2000-2.

The assessments also highlighted marked ethnic differences. A quarter of the black Caribbean and black African children who took the school readiness assessment were delayed in their development, compared with only 4% of white children.

Bangladeshi and Pakistani three-year-olds recorded relatively low scores on both tests.