Counties where the effects of the Education Bill proposals are expected to be swiftest include Cambridgeshire, Essex, Norfolk and Surrey, where grant-maintained schools predict that they can build up sixth forms of 200 students within two years and 400 within 10.

The Bill opens the door for GM schools to start sixth forms without Government approval. Tories expect to promote a flood of opt-out applications from local education authority schools and a resurgence of selection post-16. This would mirror the Bill’s proposals for more selection pre-16.

While LEA chiefs say they do not believe their schools will opt out in great numbers, they say legislation would reverse post-16 reorganisations carried out during the past decade. One chief education officer said: “Politically, this will be used as a weapon against us by those wanting approval for new sixth forms.”

Pauline Latham, chairman of the GM Schools Advisory Committee, told The TES: “It is a sensible extension of GM status to provide 16-18 education. Where sixth forms have been removed right across the board for sixth-form colleges, they will want them back. Also, schools doing well academically will want to extend the provision they have.”

However, many schools and colleges doubt that the Bill will have the sweeping effect Conservatives hope for. Heads in LEAs such as Staffordshire, Oldham and parts of Essex, including Braintree, say that consortia and other co-operative arrangements have created greater choice at A-level than any go-it-alone policy could give.

But they admit that it needs only one school to pull out for an avalanche to begin. Howard Clarke, chairman of the Sixth Form Colleges Association, said: “Most of our members can identify one or two schools locally with sixth-form proposals on a covert agenda.” He attacked the Bill as “politically motivated and educationally unsound”, arguing that schools had an in-built financial advantage which ministers had repeatedly failed to even out.

Figures from the Funding Agency for Schools suggest that schools opening sixth forms in competition with colleges are struggling to attract students during their first year of operation.

Only 14 new sixth forms opened last year, the FAS figures show. A further 13 have been given the go-ahead since last November, seven in the GM sector and six in local education authority schools. Nine applications were rejected.

Just two GM schools which opened sixth forms in September 1995 had more than 100 sixth-formers last year. Seven had between 50 and 100, while five had fewer than 50. The smallest sixth form contained just 24 students, the FAS data shows.

Two out of every three applications for sixth forms approved by the DFEE are made by a GM school. Since February 1994, when new guidelines were published by former education secretary John Patten, 56 proposals have been approved (36 from GM schools) and 42 rejected (27 from GM schools).

The lack of capital funds for expansion is also a problem. The ailing private finance initiative is unlikely to create enough cash. Only those who can afford to borrow hugely against assets based on big expansion promises have any chance, the schools admit.

Comberton Village College, an 11-16 Cambridgeshire school, expects to borrow Pounds 500,000 on the back of plans for a 200-student sixth-form within one year. Over 95 per cent of pupils stay on at 16 but have to travel up to four miles to the sixth-form college.

Mrs Shephard rejected an earlier application for a sixth form, arguing that there were sufficient post-16 places already. But Ros Clayton, head of Comberton, said the school would reapply if the legislation fails.