The gap between Wales's top-performing GCSE pupils and those in the rest of the UK has widened for the fifth year in a row, figures released yesterday reveal.
In Wales, 66.5 per cent of entrants achieved A*-C grades compared with 69.8 per cent nationally - a gap of 3.3 percentage points.
Between 2001 and 2005, Wales outperformed the rest of the UK in this measure, and five years ago the gap was just 0.1 percentage points.
The figures provide further evidence of underperformance in the education system a week after the A-level results revealed a drop in the percentage of students gaining A* and A grades.
Derec Stockley, director of examinations and assessment at exam board WJEC, said the trend was a "growing concern".
"This is a worrying gap because it wasn't there historically, and it's something we also saw in the Pisa (Programme for International Student Assessment) results," he said.
"The minister has picked up on this and has come up with his agenda for improvement focusing on literacy and numeracy, which are so important because problems with literacy prevent students from accessing qualifications in all subjects."
However, Mr Stockley also suggested the trend may be exacerbated by independent and selective grammar schools in England, which he said are claiming many of the top grades.
"Statistically, that sector is not significant in Wales, and I think if we were to compare like with like we would see more similar results," he said.
The overall GCSE pass rate (grades A*-G) in Wales was identical to last year at 98.7 per cent, and there were slight increases in the proportion of pupils gaining A* and A grades.
However, the number of entries for GCSE full courses was down by 4.7 per cent, which examiners said was a reflection of both a fall in the age group population and a move to alternative vocational qualifications such as BTECs and OCR Nationals.
There were "unusually large" changes in the numbers of entries for some subjects.
Entries for the sciences showed encouraging increases, with biology up 11.3 per cent, chemistry 13.4 per cent, and physics 11.4 per cent, continuing a six-year trend.
However, the number of entries for all modern foreign languages fell, including Spanish, which had seen increased entries in recent years.
Girls continue to outperform boys generally, and the gap in performance between the sexes has widened slightly at A*, A*-A and A*-C, despite narrowing last year.
Education minister Leighton Andrews said: "The pass rate for GCSE remains extremely high at 98.7 per cent and it's encouraging to see more entries in Wales achieving the higher grades again this year.
"I was also particularly pleased to note the strong performances across the sciences and in maths given the importance of these subjects to meeting the future needs of industry."