Scotland has fallen down international education rankings for core school subjects, according to a major world study.

The survey of 15-year-olds found Scotland's performance in reading, maths and science had declined relative to other Organisation for Economic Cooperation and Development (OECD) countries, the 2015 Programme for International Student Assessment (PISA) shows.

Scotland was ranked above the OECD average for reading and science in the 2012 study, and average in maths, but is now similar to the average for all three subjects.

Twelve countries performed better than Scotland, including England, Australia, Estonia, Finland and Slovenia.

Scotland lagged behind Estonia, Finland and Slovenia again in Maths. The country was also trailing Norway, Belgium, Ireland and Poland in the rankings.

There were also 13 countries with higher scores in reading, including Estonia, Poland and Slovenia, as well as Ireland, Finland and Norway.

Education Secretary John Swinney said the results made for "uncomfortable reading" and showed the need for reform of Scotland's education system.

The OECD research surveyed about 540,000 students worldwide in 2015 and uses a points system to rank 72 countries against each other, also looking at countries such as Scotland which are part of larger states.

Against its own 2012 ratings, Scotland fell 13 points in reading, seven points in maths and 16 points in science.

Swinney said: "There is great strength in Scottish education but these results underline the case for radical reform of Scotland's education system.

"The results undoubtedly make uncomfortable reading but they contain a plain message: 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."

He added: "Both sets of figures tell us the same thing. Reform is essential.

"That is why last year we launched a comprehensive programme of reform, based firmly on the independent findings of the 2015 OECD review of Scottish education.

"It is by carrying through on these reforms - no matter how controversial - that we can make Scottish education world-class again."

Keir Bloomer, chairman of the Commission on School Reform said: "Scotland has taken part in every PISA study since 2000.

"In 2000, its results were well above the OECD average in all three subject areas. Its performance is not now above average in any subject area.

"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."

The Scottish Secondary Teachers' Association (SSTA) said the figures were "no surprise" but highlighted responses showing Scotland is ahead of other OECD countries in teacher-pupil relations.

SSTA general secretary, Seamus Searson, said: "These figures come as no surprise. SSTA members have been concerned about the effect workload and assessment pressure are having on teachers and youngsters for a while which is why we are now engaged in industrial action."