Many children from disadvantaged backgrounds are already up to a year behind more privileged youngsters educationally by the age of three, a major study published today has revealed.

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.

A second "school readiness" assessment measuring understanding of colours, letters, numbers, sizes and shapes that was given to more than 11,500 toddlers 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.

Scottish children fared well in the assessments conducted on behalf of the Centre for Longitudinal Studies (CLS), which is based at the Institute of Education, University of London. They form part of the Millennium Cohort Study which is tracking more than 15,500 children born 2000-2.

While Scots children were three months ahead of the UK average in language development and two months ahead in "school readiness" there was still concern expressed at the findings yesterday among parents north of the border.

Brian Williamson, 42, a health service worker from Penicuik, Midlothian, said: "It has to be a worry that there can be such a big divide at an early age of three. I suppose it emphasises the responsibilities parents have for their children's education."

Mother-of-one Karen Hornal, 27, of Corstorphine, Edinburgh, said: "I am usually sceptical on these kinds of surveys making generalisations but it does seem a very large gap for children so young.

"I don't have a degree but my son is coping very well at nursery and compares favourably with classmates."

The assessments also highlighted that girls did better than boys on average. They were found to be three months ahead on both measures.

Professor Heather Joshi, director of the CLS, said: "Our Millennium Cohort Study shows that although many children who are living in difficult circumstances are doing well, too many are lagging behind their more advantaged peers in cognitive development.

"They are also more likely to suffer disability and ill-health and to experience more problems with vision and hearing, as well as asthma and other long-standing conditions, chronic infections and injuries."

The assessments also highlighted marked ethnic differences. A quarter of the black Caribbean and 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.

Dr Kirstine Hansen, the study's research director, accepted the assessments might not be a fair indicator of minority ethnic children's current or future ability.

She explained: "Before drawing firm conclusions we will need to investigate the circumstances in which the assessments were done, allowing for whether children lived in homes where English was not the main language spoken.

"There may also be cultural differences in their readiness to attempt tasks or engage with an unfamiliar visitor."