The opening of up to 100 new 14-19 technical schools by 2015 risks damaging education in existing secondaries, a heads' association has warned.
Brian Lightman, general secretary of the Association of School and College Leaders (ASCL), said he was worried about the consequences of opening university technical colleges (UTCs) in areas where there are already surplus places.
Lord Baker, former Conservative education secretary, told The TES this week that widespread interest from higher and further education, local councils and groups of teachers means the 600 to 800-pupil UTCs he has been promoting have "ceased being experimental".
"This has now become a movement," he said. He confirmed that some of the UTCs he expects to open would create extra secondary places in areas that already have surpluses - a prospect that alarms the ASCL.
Mr Lightman said: "If these are new additional schools in areas where there is no basic need (for places), that is going to compromise the quality of education in other schools because it will take resources away from them."
Lord Baker said: "I know that is their concern. We say simply that the catchment area of UTCs is wide so that it doesn't affect any secondary school too much."
UTCs are sponsored by universities and focus on technical education. There are likely to be many more than the 12 originally pledged.
Lord Baker has predicted as many as 100 by the end of this Parliament and this week he said talks were taking place with more than 45 groups.
The news came as another former education secretary, Baroness Morris, became the latest educationalist to call for England to follow most other industrialised countries and allow pupils to choose between academic and vocational routes at 14.
The Labour peer said GCSEs should be replaced with progress tests at 14 to guide pupils on what to study next.
But Mr Lightman and the NUT both oppose the idea of vocational and technical education being moved to separate schools. Some believe it could mean the return of widespread state school selection, but Lord Baker said they had nothing to fear from UTCs.
"We can't select our pupils," he said. "There is no selection process, no exams or anything of that sort. People simply apply."
He said there was a large demand for UTCs from employers. The first UTC is sponsored by JCB and is across the road from the company's HQ in Rocester, Staffordshire.
But Jim Wade, principal of the JCB Academy, said: "Students are not coming here to be prepared to work in a factory ... We are not providing a vocational education, but a technical education, which in my opinion is as intellectually demanding as something like Latin or Greek."
SPANNER IN THE WORKS - BAC SETBACK FOR FIRST UTCs
Students at the first university technical colleges (UTCs) will not have their achievements recognised under ministers' new benchmark for secondary school success, it has emerged.
The English Baccalaureate introduced to league tables this week demands that pupils achieve a portfolio of GCSEs or IGCSEs including either history or geography. But the curriculum at the JCB Academy in Rocester, Staffordshire, does not allow either subject to be studied.
Principal Jim Wade said he was aware his college, which focuses on engineering, stands to miss out in the English Bac tables when his first students take GCSEs in 2013. But he added: "Like any school we need to offer the curriculum that works in the best interests of our students."
Mr Wade said he hoped the "powers that be" would accept the alternative "tech Bac" that Sir Mike Tomlinson is developing for UTCs.
Lord Baker, UTC promoter, said the tech Bac would include history, but added it would be the history of science, invention and economic development.
There would also be a foreign language but it would be "German for engineering, not Goethe" or "French for business, not Moliere". "That may not be at the GCSE level," Lord Baker added.