The proportion of schools in special measures or needing serious improvement has doubled under Estyn's new inspection regime, TES Cymru can reveal.
Over the last six-year cycle, just 4 per cent of schools were put into the top two categories for causing concern - but after the first 100 inspections under the new common inspection framework the figure stands at 8 per cent.
The news comes as a major Estyn report published this week highlighted that one in three schools in Wales is not reaching high enough overall standards.
In her annual report, Ann Keane, HM chief inspector of education and training in Wales, said that while standards have improved since 2004, progress has been slow, and only a handful of schools are consistently outstanding.
The report also confirms that the Assembly government missed many of the 2010 targets set out in the 2006 Learning Country: Vision into Action document, including attendance, class sizes and teacher assessments for 11 and 14-year-olds.
Teaching unions and politicians called the report "damning" and demanded urgent action. However, early findings from Estyn's new inspection framework are also causing concern among heads and teachers.
Since September last year, around 100 schools and educational settings have been visited by inspectors under the new regime, which was originally billed as a "lighter touch" than the previous system.
The inspectorate anticipated that around 30 per cent would need some form of follow-up activity, but the current figure is more than 40 per cent. The number currently requiring the most serious interventions is double the average for the previous six-year inspection cycle.
Ms Keane told TES Cymru the shift of focus from subjects to skills could explain the unexpected increase.
"Maybe we were missing some of this before," she said. "These are skills that every school should be teaching across the curriculum.
"By focusing on skills we are not setting different standards - I hope it will be a motivating factor and a way of helping schools focus more on how they teach skills."
However Rex Phillips, Wales organiser of the NASUWT teaching union, accused the inspectorate of "moving the goalposts".
He said: "This has caused a great deal of consternation among our members. Schools that were previously getting grade ones are being downgraded under the new regime and it looks like standards have fallen when they haven't."
Ms Keane said it is too early to say whether the trend is likely to continue, but she is satisfied the inspections carried out so far have been robust and said there will be no changes to the guidance given to inspectors.
She said Estyn is closely monitoring the situation and its staff are poring over each report to make sure the inspectors' judgments are justified.
- Original headline: Proportion of `failing' schools doubles under new Estyn regime