The Welsh government needs to "urgently intervene" in a council's plans to reduce English usage in its schools, the leader of the Tories in the Senedd said.
Under Cyngor Gwynedd's proposals, at least 70% of the curriculum would be taught in Welsh, with English-medium streams ending in secondary schools.
Overall there was support for the plans at a scrutiny meeting on Thursday, but Tory Darren Millar described it as "fundamentally wrong", and even suggested the UK government should get involved.
The Welsh government said local authorities were responsible for planning in schools.
Writing on the Conservative Home website, Millar argued that the proposal is "not only harmful to education but also unfair to teachers and divisive for communities".
He also criticised it by calling it part of a broader trend of inward-looking policies in Gwynedd.
Millar called on Education Secretary Lynne Neagle to "urgently intervene" and warned, "If the Welsh government fails to protect citizens' rights, the UK government must step in".
During Thursday's meeting, some Gwynedd councillors felt the draft policy lacked boldness, while others expressed concerns about the inadequate monitoring of progress.
However, there was broad support for the aim of expanding Welsh-language education.
In response to Millar's opposition, Gwynedd's education portfolio holder, councillor Dewi Jones, called his stance "disappointing" and "very unfortunate".
