The assembly government has strongly denied that the growing attainment gap between Wales and the rest of the UK is linked to school funding.
Last week the overall pass rate for A-levels in Wales dropped for the first time this decade, down from 97.6 per cent to 97.1 per cent, while the percentage of students gaining A grades also fell from 25 per cent to 24.4 per cent.
It means the A-level performance gap between Wales and the rest of the UK has widened for the second successive year.
There are also widespread fears that Wales could slip further behind the rest of the UK in terms of attainment when the results of this year's Pisa international student assessments are published in December. Wales had the worst results of the four home nations in the previous Pisa tests in 2006.
Critics said the funding gap between Wales and England - which currently amounts to pound;527 a pupil - is having an effect on attainment.
Paul Davies, the Conservative shadow education minister, said: "I'm concerned we are slipping behind other parts of the UK. The funding gap has widened and now the attainment gap has widened too, so there must be a correlation.
"The Assembly government has to take responsibility here and it must fulfil its promises to reduce bureaucracy and move more cash to the front lines."
Speaking about the A-level results, David Evans, secretary of teaching union NUT Cymru, said: "It's not a disaster now - it is a concern. We're not saying that the reason for it is solely because of the funding, but it is a reason."
But the Assembly government strongly denied any link. A spokeswoman said: "School funding in Wales has increased by 71 per cent since 19992000. We have made it clear that we want to ensure that more funding reaches the education front line and that's why we commissioned Price Waterhouse Coopers to review the cost of administering education in Wales."
She added that the Assembly government was committed to spending 1 per cent on top of the annual block grant for education that it receives from Westminster.
The Assembly believes the results of the skills-led Welsh Baccalaureate qualification should be considered alongside A-level results, as successful candidates have gained the equivalent of an A grade at A- level.
"Taking this into account significantly alters the profile of Wales' young people's comparative success rates," said the spokeswoman.
Derec Stockley, director of examinations and assessment at the WJEC exam board, called this year's A-level results a "slight blip" but admitted they were still a concern.
"This growing attainment gap with the rest of the UK, which is something we know from the Pisa results, is a phenomenon peculiar to Wales, which is more worrying.
"We are proposing that we meet curriculum leaders and heads as soon as possible to discuss what the A-level data tells us and to look at what we can do to help."
The WJEC is also planning to offer GCSE and A-level-themed inset days for schools on a regional basis from 2011.