Top-performing specialist schools will be allowed to open new sixth forms under government reforms published this week.
New guidance says schools with a specialism in vocational subjects and those requiring no additional building work will be given priority.
The move follows an earlier announcement in the Government's education white paper that it should be easier for the best secondary schools to add a sixth form - part of the drive to expand choice for 14 to 19-year-olds.
But the proposals were instantly seized on by critics who said opening more school-based sixth forms would rob existing sixth-form and further education colleges of students, placing their futures under threat.
Long-term population forecasts already show that the number of 16 to 19-year-olds will fall sharply from 2009 onwards.
But the Government said increasing the number of school-based sixth forms would simply attract more teenagers into post-16 education, matching higher participation levels now witnessed in countries such as Germany and France.
This week's guidance says high-performing specialist schools with a second vocational specialism would be considered first. Under existing rules, the most successful specialist schools have the opportunity to adopt additional specialisms, including the vocational option, to create greater balance in local school provision.
Schools with close links to existing FE colleges, including centres of vocational excellence (Coves) will also be favoured.
The document says guidance to local school organisation committees, which have the final say on new sixth forms, will be amended to favour vocational specialist schools, and the timetable for approving applications will be speeded up. It also says schools that do not require additional building work for a new sixth form should be given top priority.
The Association of Colleges said this week that it predicted as many as 80 specialist schools will be able to develop new sixth forms under the initiative.
But it warned that the schools were entering a crowded marketplace, and said that competition for students was already intensifying with the opening of 200 academies, almost all of which are designed for pupils aged up to 18.
Julian Gravatt, director of funding and development at the AoC, said: "This is coming at a time when the number of 16 to 19-year-olds is going to start declining soon as well.
"Small school sixth forms cannot offer the breadth and range that colleges can."
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