The only Welsh-medium secondary school in Wrexham county has been let down by the council "on purpose", a former Ofsted inspector has claimed.
John Morris' comments come after an Estyn report found aspects of Ysgol Morgan Llwyd, Wrexham, were "in need of significant improvement".
A chair of governors letter to parents said it had a "robust plan" to address the issues.
Wrexham council said it was giving the school "targeted and specific support".
In 2014, the school was assessed as "good" after an inspection by Estyn but four recommendations were made for improvements.
Progress in each of these areas was found to be "strong" or "satisfactory" in March 2017.
But the monitoring report found that performance in Key Stage 4 was a "cause for concern" and that there had been "insufficient progress".
Mr Morris, a former head teacher at the school, claims that its leadership should have received more support.
"The local authority has let the school and teachers down," he told the Newyddion 9 programme.
"I consider the county council to have allowed the demise of the school on purpose because of the large numbers of children who want to go to that school. It's an embarrassment.
"The council know that they have to refuse access to 50 children a year to Welsh medium schools.
"It's an absolute disgrace that they haven't given enough support to the teachers at Ysgol Morgan Llwyd."
Councillor Hil Wynn, lead member for education, said: "Through the local authority and GwE - the regional school improvement service - targeted and specific support has been provided to Ysgol Morgan Llwyd in order to assist the school in addressing the recommendations of this report.
"This will continue as the school works to further raise standards.
"The council is committed to Welsh language education as is demonstrated in the Welsh in Education Strategic Plan and as evidenced by the growing demand for Welsh medium education in the primary sector."
In a letter to parents, chair of governors Aled Roberts said Estyn's report was a "recognition of the significant improvements that have occurred" and "fully recognise the rapid and strong changes the senior management team have implemented".
"There is a robust plan in place that addresses the concerns expressed and we are confident that the pupils at Ysgol Morgan Llwyd will receive the best provision in all aspects of school life," he added.