Pupils making the transition to secondary school this week are leaving primary with poorer results than their predecessors. And the slide in core subject performance has led to calls for a review of the primary curriculum.
Provisional national curriculum assessments show the percentage of 11-year-olds achieving the standard expected for their age (level 4 or above) went down 0.7 percentage points in English to 78.6 per cent.
Results were also down one percentage point in Welsh and science (to 75.3 and 85.5), although performance at maths rose by the same margin to an all-time high of 81 per cent.
Gender breakdowns show that more than a quarter of boys started secondary school this week without the literacy skills needed to access the curriculum, as did 15 per cent of girls. Overall, girls continue to outperform boys, particularly in Welsh and English.
Assessment results were also down for seven-year-olds in every subject except science - which remained unchanged from last year, with 89.4 per cent achieving level 2 or above.
An Assembly government spokesman said primary children in Wales still do better than their contemporaries in England at key stage 2, and that this year's 11-year-olds had achieved the country's best-ever maths results. It is also hard to improve on the "consistently high" results at KS1.
She added: "There will always be differences in the performance of different groups of children. These are bound to be reflected in overall performance figures.
"Strategies and policies to improve standards at KS2 are not quick fixes, and it often takes years to reap the rewards of some of the excellent work undertaken by our teachers and schools."
But she said changes to assessment arrangements in Wales, particularly the removal of tests and optional external marking at KS2, may have had an effect. There have also been changes to data collection, and independent schools are not in the 2006 figures.
David Evans, secretary of the National Union of Teachers Cymru, also cautioned against over-reacting to a fall in results in just one year. "It would be a concern if this continued into another year, but there is no need for wholesale panic at this stage," he said.
The Assembly government's 2007 target, of 75 per cent of 11-year-olds achieving the expected level in all three core subjects (EnglishWelsh, maths, and science), remained realistic, he added. This year's core subject indicator was 74.1, down from 74.3 in 2005. In 1998, it was 52 per cent.
Geraint Davies, policy officer of teachers' union the NASUWT Cymru, also warned against focusing on just one year's results.
But he said: "There is still room to look at the primary curriculum. The demands we make on pupils are still unacceptable. There is a need to streamline the curriculum to concentrate far more on basics."
Dr Philip Dixon, director of the Association of Teachers and Lecturers Cymru, said: "A review of the curriculum is needed. We are moving from a knowledge-based to a skills-based one, and we need to move faster on that."
John Evans, president of the National Association of Head Teachers Cymru and head of Marlborough Road juniors, Cardiff, said: "If we adopted a skills-based model, teachers would still have to meet the requirements but they would have more autonomy in terms of content. It would be more interesting for children - they wouldn't be doing the Romans every year."
It was unrealistic to expect year-on-year improvements, he added. Teachers are working as hard as ever, but variations also arise because some cohorts have more children with SEN statements.
The publication of provisional teacher assessments for this year's 14-year-olds has been delayed because of data problems.