STANDARDS of reading and science in Scottish schools are declining while performance in mathematics is stagnating, according to a major international survey.

The 2015 Programme for International Student Assessment (Pisa) concluded that Scotland's performance was now "average" compared to other developed countries in all three disciplines.

In 2000, when Scotland first took part in the OECD survey of 15-year-olds, its performance in reading, science and maths were all above average.

Countries which have now moved ahead of Scotland in science include England, New Zealand, Slovenia, Switzerland and the Netherlands.

In reading, a greater number of countries performed significantly higher than Scotland with Australia, Germany, the Netherlands, New Zealand, Norway and Slovenia all moving ahead.

While performance in mathematics has not declined since 2012 it is still lower than it was in earlier years and the number of countries statistically above Scotland is the highest it has been since 2003.

The stark findings have thrown the spotlight on the implementation of the controversial Curriculum for Excellence (CfE) reforms which were introduced in 2010.

The changes were supposed to improve basic standards and empower teachers, but CfE has been plagued by claims of unnecessary bureaucracy, increased teacher workload and confusion about its aims.

Since then the Scottish Government has announced a raft of fresh reforms which will see national standardised tests in primaries, new educational regions to help run schools and greater power to headteachers.

Pisa also comes as a blow to efforts to improve science teaching in Scotland after concerns the subject was not being taught in way that pupils considered relevant to society.

John Swinney, the Education Secretary, said the Pisa results underlined the case for radical reform of Scotland’s education system.

He said: "The results undoubtedly make uncomfortable reading, but they contain a plain message that we must continue to make the changes that are necessary to strengthen Scottish education.

"We must recognise that while Pisa is only now being published, it dates from the period in which our own statistics on literacy and numeracy were published and prompted our current programme of reform. Both sets of figures tell us the same thing. Reform is essential.

"It is by carrying through on these reforms, no matter how controversial, that we can make Scottish education world-class again and this requires an unwavering focus on improving Scotland’s education system."

Keir Bloomer, a member of Reform Scotland's advisory board and chair of its Commission on School Reform, also called for radical action.

He said: "Scotland's performance is not now above average in any subject area and it is no longer credible to describe Scotland’s education system as world leading.

"There is a critical and urgent need to examine how Scottish education is run and the Scottish Government's stated intention to empower teachers, parents and schools must be matched by action."

However, teaching unions urged caution arguing the results affected 15-year-old at a time when CfE had not been fully implemented and schools were being challenged by funding cuts.

Larry Flanagan, general secretary of the Educational Institute of Scotland teaching union, called for schools to be properly resourced.

He said: "These tests were taken by 15-year-olds almost 18 months ago and this is precisely the area which we have highlighted as the least well implemented area of CfE.

"The need for continued investment in education is made clear, however, by this report and the Scottish Government needs to ensure schools are properly resourced and supported."

Euan Duncan, president of the Scottish Secondary Teachers' Association (SSTA) called for a period of stability to allow CfE to bed in.

He said: "All the changes that have taken place in recent years have added tremendously to the pressure on teachers and youngsters and reductions in support staff and supply teachers have taken teachers away from their core function.

"There is no magic solution to improving the downward Pisa performance trend, but good starting points will be to provide teachers with very clear aims and sufficient resources with which to achieve them."

The 2015 Pisa results showed Scotland’s scores were similar to the OECD average in science, maths and reading. In the 2012 survey Scotland was above the OECD average in reading and science and at the average in maths. Scotland’s performance was lower than 2012 in science and reading, and similar to 2012 in maths.

Girls outperformed boys in reading, but boys and girls had similar performance in maths and science.

Socio-economic background had a similar effect on performance compared to 2012, but is still reduced compared to the results of the 2009 survey.

Pupils in Scotland are generally more positive about the value of learning science at school than is the case across the OECD. Classroom disruption is generally lower than average and relationships with teachers more positive.

Around 540,000 pupils participated in the study worldwide from the 35 member states of the OECD and 37 partner countries.

Singapore topped the rankings in all three subject areas.