The country's biggest academy sponsor has been plunged into fresh turmoil after it emerged that another of its schools has been judged "inadequate" by Ofsted.
Stockport Academy, sponsored by the United Learning Trust (ULT), has been told that it needs "significant improvement" to address poor standards.
It is the third ULT academy to be described as inadequate by inspectors in less than a year and is the latest in a line of significant setbacks for the sponsor.
Sheffield Park Academy was put into special measures last September after a highly critical Ofsted verdict. This followed Sheffield Springs Academy being given a notice to improve in June.
At the end of last year ULT, a Christian charity, pulled out as the backer of two further planned schools in Oxford and Northampton in the wake of concerns over the group's performance.
Ed Balls, then schools secretary, described the situation in Sheffield at the time as "unacceptable" and expressed his "very serious concerns" to ULT, which sponsors 17 schools in total.
Results at Stockport Academy deteriorated last year, with just 25 per cent of pupils achieving five good GCSEs, including English and maths, down two percentage points on 2008.
The Ofsted report cites numerous concerns about the school, which opened in 2007, including the development of pupils' basic skills, especially at key stage 3, and poor attendance.
"Learning and progress are inadequate at key stages 3 and 4," the report finds.
"Too many students have made insufficient progress from the levels they attained in national tests at primary school."
The quality of teaching is a concern, with not enough of it being of a high standard, inspectors said. The school has enjoyed some success at key stage 4, with almost all students leaving with at least one qualification and "many more" going into further education, training or employment.
Kathy August, ULT's deputy chief executive and principal of the high-performing Manchester Academy, said Stockport was already receiving additional "support, capacity and attention that is paying dividends".
The school was expecting improved GCSE results this year, she added.
On the issue of ULT's performance more widely, Ms August said that it had taken on schools in challenging areas that other sponsors would not have backed.
"By the nature of the schools we have taken on, we have had a bigger job than some other sponsors," she said. "But we are making sure that our academies are as good as they can be, as quickly as we can get them there."
John Bangs, head of education at teaching union the NUT, said: "This is another example of the illusion that by conferring academy status on schools they automatically improve. Ofsted's ridiculous expectations that schools improve in a year apply equally to maintained schools and academies. All schools in tough areas need support."
ULT has previously been criticised by John Dunford, general secretary of the Association of School and College Leaders, for the high turnover of heads at its schools.
As revealed in The TES last year, more than half of the academies it was then running had lost their principals within two years of the schools opening.