English, Welsh and Maths GCSE results are expected to be lower in Wales this year than previously, the body overseeing qualifications has said.
The predicted decline is being blamed on schools entering more pupils for the exams a year earlier than intended.
Education Secretary Kirsty Williams warned pupils capable of high grades could be missing out by sitting the exams early and not being re-entered.
She told BBC Wales she may stop schools entering pupils for exams a year early.
Speaking on the Sunday Politics Wales programme, Ms Williams said she would consider the matter after an investigation reports back to her in the autumn.
For GCSE English Language, entry data shows a substantial number of pupils taking the qualification this summer while still in year 10.
There are slightly more than 21,000 year 10 entries – approximately 65% of all year 10 students in Wales.
Exam regulator Qualifications Wales said that as a result “we expect that this will mean that the overall results this summer will be lower than in previous years”.
Ms Williams said: “What I’m concerned about is that children that, perhaps had the potential to get an A* and A or a B at the end of a two year course end up having to settle for a C because they do it early and they’re not re-entered again.
“I want children to fulfil their potential in school. I want early entry to be only for the children who will benefit from it.
“When I see such large numbers as are being reported as being entered, that’s something I am concerned about.”
This year, for the first time, pupils have sat GCSE English and Welsh exams which are unique to Wales.
Ms Williams said the new qualifications were more rigorous and this could also account for the expected drop in results.
However, last October, Qualifications Wales wrote to schools about the new qualifications and insisted: “There is a well-established approach to maintaining stability in outcomes when a new GCSE or an A level is introduced, known as ‘comparable outcomes’.
It said the aim was to ensure pupils taking new qualifications were “neither advantaged nor disadvantaged” compared to those who took the previous exam, and that the approach “works well in situations where one specification is replaced with another”.
In May, Welsh Government Director of Education Steve Davies told AMs some schools were “gaming” the exam system, that officials were “concerned” about the volume of early entry and the government wanted to take action this autumn.
Earlier in the month, Ms Williams expressed concerns the focus on raising GCSE attainment to C grade had led to “unintended consequences”, with some children put in for early entry to bank a lower qualification instead of potentially reaching higher.
This is an issue which has been on the Welsh Government’s radar for a number of years.
In 2013, the then Education Minister, Huw Lewis, warned he would intervene to prevent schools “gaming the system” in the future if they did not stop voluntarily.
In England, the number of pupils being entered early for their Maths GCSEs fell sharply in 2014 when the UK government made changes to how school league tables were calculated so that only the first entry counted.