Wales is falling behind the rest of the UK in top academic results because the school system is failing its most able pupils, a former Assembly government adviser has claimed.
This week's GCSE results revealed that the gap between the top grades achieved by Welsh pupils and those in the rest of the UK has widened for the fourth year in a row.
In Wales, 66.4 per cent achieved top A*-C grades compared to 69.1 per cent nationally, a gap of 2.7 percentage points. Four years ago the gap was just 0.1 per cent.
The proportion of pupils gaining A* and A grades increased slightly to 19.2 per cent, but lagged behind the national result of 22.6 per cent.
Last week's A-level results revealed that 6.5 per cent of students here gained the new A* grade, compared to 8.1 per cent nationally.
David Reynolds, professor of education at Plymouth University, said there has been too little focus on the country's more able and talented pupils.
"These results suggest we have a higher-ability problem, not a lower- ability problem as was traditionally assumed," said Professor Reynolds, who lives in Wales.
"England has had an obsession with stretching its most gifted students and they are seeing the results. Our most able and talented just aren't good enough by comparison.
"They are being failed by the system, and the Assembly government ignores these results at its peril."
But Derec Stockley, director of examinations and assessment at the WJEC exam board, disputed this.
"What's happened in England is that the independent school sector and selective grammar schools have got to grips with GCSEs and they are claiming a huge number of the top grades," he said.
"We don't have that to such an extent in Wales, and I think that's the main factor."
Mr Stockley admitted that more research was needed on why Wales was falling behind when it had traditionally exceeded national results, and said the WJEC would discuss the matter with headteachers.
Rex Phillips, Wales organiser of the NASUWT teaching union, said it was wrong to concentrate solely on the best and the brightest.
"We should not lose sight of the fact that pupils who achieve a D, E or G may well have excelled themselves," he said.
"The concentration on A-to-C grades goes against the ethos of the GCSE system, which was introduced in order to create a level playing field."
The overall GCSE pass rate in Wales increased by 0.2 per cent to 98.7 per cent this year, the same as the national pass rate.
Experts warned that results may have reached a "glass ceiling", with little room for further improvement.
Carwyn Jones, the Assembly government's first minister, congratulated pupils, and said: "The young people of Wales have performed excellently and they should be proud."