There has been a significant decline in literacy standards among Irish 15-year-olds over the past ten years, according to a highly regarded international education survey.

The OECD's latest PISA study ranked the reading ability of Irish 15-year-olds in 17th place out of 39 countries, compared to 5th place in 2000.

Tánaiste and Minister for Education Mary Coughlan has said she is disappointed with the outcome.

Every three years the OECD measures the performance of 15-year-olds in reading, maths and science across dozens of countries.

In its latest survey, Irish students fared well in science, once again significantly above average. Achievement has fallen in maths, from average to below average.

However, the really significant decline is in reading - from 5th place among 39 countries ten years ago to 17th place now.

The Department of Education commissioned Irish and Canadian researchers to find out why. Their research says changes in the school-going population may have had some influence.

There are more migrant children, fewer early school leavers and a greater inclusion of students with Special Education Needs.

But both groups say these and other factors cannot account fully for such dramatic change.

The Department of Education says it is taking the OECD findings seriously but it is also urging caution and points to other studies of the same student group which showed no decline in ability.

The INTO has said the results will damage the international reputation of Ireland's education system.

However, the primary teachers' union said the results should be treated with caution as all other evidence shows that Irish literacy and numeracy standards are stable.

The INTO said it was important to note that experts from the Educational Research Centre concluded that changes in the curriculum at Primary level cannot explain the decline in performance on reading literacy between 2000 and 2009.