Calls have been made for an inquiry into key stage 3 education after new statistics revealed a record attainment gap between 14-year-olds in Wales and England.
Official figures for last year showed that Welsh KS3 pupils continued to perform worse in English and maths than their counterparts in all regions of England, and only pupils in London performed worse in science.
The difference in English results was the greatest since devolution in 1999, with 69 per cent of 14-year-olds in Wales achieving the expected level compared to 77 per cent in England. In 2000, the difference was just 1 percentage point.
Overall, the percentage of 14-year-olds in Wales attaining the expected level in the three core subjects of English, maths or science remained the same as 2008.
The statistics showed that pupil attainment between the two countries is similar at KS1 and by KS2 Welsh pupils are actually outperforming those in England, only for the results to drop off at KS3.
David Reynolds, professor of education at Plymouth University, said: "Problems begin at secondary rather than primary level and the Assembly government really needs to conduct an inquiry into what's going wrong in KS3.
"It's quite extraordinary; the difference between Wales and England is getting worse and worse."
Professor Reynolds, who is a former Assembly government adviser, blamed a lack of funding and poor transition arrangements.
"We know schools in Wales are underfunded and, simply put, if you have got more money you can do more," he said. "With transition, the adjustments of going from a small primary to a large secondary may be taking their toll on pupils."
Although the statistics showed that Welsh pupils performed better than English pupils in all core subjects at KS2, the reliability of those figures was recently thrown into question.
In a report to the Assembly government, Estyn said teacher assessments of 11-year-olds are often inaccurate and unreliable.
Its inspectors found huge inconsistencies between individual primaries in the levels they awarded to pupils at the end of KS2, and said that more robust systems in place at KS3 are resulting in greater accuracy.
An Assembly government spokeswoman said: "While we are keen to learn and benchmark against those near and far away, it's important to acknowledge that the curriculum and assessment now differs between England and Wales.
"Although we acknowledge that performance in Wales is lower than that in England at KS3, it is important to note that over time Wales has made gains for all three core subjects. We are delighted that in Wales results show improvement year on year."