Scottish secondary pupils have slipped in their results in maths and reading as other countries overtake them in a global league table of achievement.

While performance as a whole remains above the average for the developed world among 15-year-olds in Scotland, research pinpointed that attainment in maths and reading had fallen, while achievements in science remained roughly the same.

The research sampled the achievement of 400,000 secondary schoolchildren across 57 countries in a study designed to reflect the potential impact of young learners on the world's economy.

In Scotland, 101 schools and around 2700 pupils took part in the study by the Programme for International Student Assessment (Pisa), which this year focused on achievements in science.

In this subject, Scotland has slipped down the international rankings to ninth, although only pupils in Canada, Finland, Japan and New Zealand significantly outperformed Scots pupils. A slight change in the test methods may have had an impact on the final results, researchers said.

The mean score of Scottish pupils was 515, compared with an average of 500 and ranks alongside Australia, Germany, England and Wales in terms of achievement. Researchers also found that there was a strong batch of high achieving science students in Scotland, with just over 10% of those tested reaching level five - the second highest grade - compared with the international average of 7.7%.

But overall, Scotland's performance in science has not changed while other countries have improved in this field.

Maths results still scored above the international average of 498 with a grade of 506, but researchers found that the performance of Scottish students has diminished in this subject.

The figures rated Scotland as 12th in maths, with six countries significantly further ahead. That compares with just three countries in 2003.

"This is a significant decline," the report stated.

While performance in reading was rated as good overall, results have again declined.

More countries are beating Scotland in reading than ever before and the average mean score for reading attainment has slipped by 17 points. To compare, most other countries' scores remained the same with Korea increasing by 22 points and Norway dropping by 22 points.

The gap between the poorest readers and the best readers has also increased slightly.

Scotland ranks alongside Australia, Denmark, France and England in terms of pupils' reading achievements, though Canada, New Zealand and Ireland scored significantly better, according to the research.

Schools Minister Maureen Watt said: "While it is clear that many of our pupils are still performing above the OECD average, we have inherited a situation which shows our international ranking has fallen since the last survey in 2003. The study also shows that the impact of a disadvantaged background has a greater effect on how our students perform than many other countries.

"That's why this government is investing in early intervention to ensure our children get the help and support they need to spot and deal with problems before they start having an impact.

"And we are developing the Curriculum for Excellence which will improve standards for all children in our schools."

Judith Gillespie, of the Scottish Parent Teacher Council, said that Scotland showed "satisfactory" performance in a study which incorporated many hidden variables.

Ms Gillespie said: "I think all these international comparisons have so many caveats, you can't take account of so many factors, such as which particular schools took part and which pupils were there on that particular day. Given the nature of this type of study, one would expect a degree of of mobility."

Ronnie Smith, general secretary of the EIS, described the report as a "largely positive snapshot" of Scottish education but warned against complacency. He said: "This report also highlights that we must continue to invest in our education system if we are to continue to compete, in both educational and economic terms.

"There are already signs that other countries are improving faster than Scotland in areas such as mathematics and reading."