Five academies and free schools with 'unacceptably low' performance have received formal government warnings, starting a process that could see them taken away from their current sponsors.
The letters to City Academy Norwich, Beccles and Saxmundham Free Schools, and the Diamond and Admirals Academies in Thetford, were sent last month, and made public yesterday.
In them, regional schools commissioner Tim Coulson warned that, unless action was taken, the government could appoint additional directors, effectively taking over the schools' governance. This could ultimately lead to their sponsors being stripped of the schools.
The government has backed the creation of academies and free schools - taxpayer-funded schools independent of the local authority - to improve standards.
Colin Collis, of the NASUWT teaching union, said: 'The more time goes on, the more it can be seen the academy programme is deeply flawed, and does very little to improve the education of young people.'
The Department for Education (DfE) said academies and free schools operate under 'a strict system of oversight and accountability', more robust than for other schools, allowing it to take swift action.
The notice is the latest blow to City Academy, which has been below the government's target of 40pc of pupils gaining five GCSEs at A*-C, including English and maths, for three years, and suffered financial difficulties.
According to provisional results, 25pc of its pupils achieved the 'gold standard' this year, compared to 29pc at Saxmundham Free School, and 43pc at Beccles Free School.
In a statement, City Academy said: 'Over the last 12 months, under the leadership of our new principal, we have implemented a series of measures with the singular focus of improving our students' attainment at GCSE.'
Robert Cawley, chief executive of the Seckford Foundation Free Schools Trust, which sponsors the two Suffolk free schools, said a trust-wide GCSE improvement strategy was in place, added: 'Despite the headline figures at both schools there were some very positive outcomes, with strong GCSE results in individual GCSE subjects and some excellent individual achievements.'
The 2015 results for primary schools will be released next month.
Joyce Hodgetts, managing director of the Academy Transformation Trust, which sponsors the two Thetford primary schools, said its improvement directors have been working closely with the staff and governors and the DfE, and submitted plans to secure the required improvements, including extra staff training in English and maths.
She added: 'Although we know there are improvements to be made at both academies, pupils are improving each year. We are proud of our staff and our pupils and we will continue to work together with staff and the DfE to provide our pupils with the very best education.'
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