A major academy chain has been heavily criticised by the schools watchdog after inspectors concluded that too many pupils are not getting a decent education.
Inspectors warned that around half of the academies run by the Academies Enterprise Trust (AET) are not yet considered good, adding that some had been left to "flounder".
AET insisted that Ofsted's conclusions, which followed inspections of a dozen of its academies, did not give the true picture of progress across its 77 schools, and added that it had raised a number of issues with the inspectorate about its interpretation of the data and "potential errors of fact".
Ofsted visited 12 AET academies in June and found that only five had improved since their previous inspection. Half of those visited continue to be "less than good", the watchdog said, with one still rated as inadequate and one declining in performance since its last inspection.
In a letter to AET's chief executive, Ian Comfort, setting out the findings, Ofsted's chief operating officer Matthew Coffey, said: "It remains the case that half the academies in the Trust are not yet good. As a result, too many pupils in the Trust are not receiving a good enough education."
The inspections highlighted "key weaknesses" in the schools visited, Ofsted concluded, including low expectations of what pupils can and should achieve, pupils with "less than good" attitudes to learning and unacceptable behaviour and that classwork was not always challenging enough.
The letter warns: "Overall, there is too much variability across the Trust, with some academies left to flounder."
It added that much of the evidence found by inspectors supported a view put forward by a respondent to Ofsted's survey of AET academy leaders, who said "the Trust has grown faster than the capacity of central leadership to manage".
This survey also found that some AET academy leaders felt isolated from the Trust and did not believe that the Trust played a significant part in the development of their schools, while some leaders said they did not know how the Trust intends to ensure that every academy is good or better.
"AET has not provided effective support to all its academies," the letter concludes.
"The rapid expansion of the Trust and a lack of strategic leadership have hindered improvement. Overall, some academy leaders are sceptical that the Trust will be able to help them improve to a good or excellent standard."
In a statement, AET said it recognised Ofsted's role in monitoring standards and said it shared the watchdog's commitment to high academic performance.
But it added: "T he targeted inspection of 12 academies does not give the true picture of progress across our 77 academies. Since September 2013, 16 schools have been judged "good" and a further three "outstanding". We have raised a number of issues with Ofsted about their interpretation of the data and potential errors of fact. In particular, we are concerned that the letter intended to summarise the 12 targeted inspections places an unfairly negative slant on the more balanced assessments in the reports themselves.
"Many of the academies inspected by Ofsted have a history of underperformance and have been with AET only for a short time. Turning a school around takes time, but we are acting to ensure a rapid and sustained improvement in these academies.
"Examination results in AET schools have been improving faster on average than other schools. This is demonstrated by a six percentage point increase in our Key Stage 2 results this year - while at A-level, the number of pupils achieving A* to B at AET academies improved significantly, even as the national average dropped. We are awaiting a final analysis of the 2014 GCSE results but last year saw a 5% rise in the percentage of pupils gaining 5 GCSE passes A-C across the group."
In July, it was announced that David Hoare, who was appointed as a trustee of AET earlier this year as part of an effort to improve standards in the Trust's schools, will become the new chair of Ofsted. At the time of the announcement of his new role, Mr Hoare stood down from the AET board.
AET is the fourth academy chain that Ofsted has raised concerns about, with critical letters sent to School Partnership Trust Academies (SPTA), Kemnal Academies Trust (TKAT) and the E-ACT Trust - one of England's biggest academy organisations.
Ofsted chief inspector Sir Michael Wilshaw said: " So far Ofsted has carried out inspections of four multi-academy trusts.
"While some academy chains are performing well, I am concerned that a significant number are not doing as well as they should in order to deliver the improvements their pupils deserve. The Department for Education must ensure that there is sufficient leadership capacity in each chain to improve the performance of the constituent schools.
"These concerns about performance make it all the more important that Ofsted is given the additional power to examine the work of the head offices in these chains. This is something I hope to raise in future discussions with the new Secretary of State."
Shadow education secretary Tristram Hunt said: "David Cameron's schools policy has created an absence in oversight of education standards, leading to more children receiving substandard schooling. As a result, problems are not being spotted early enough.
"The decision to rapidly expand the underperforming Academies Enterprise Trust was taken by ministers. The ongoing refusal by this Government to allow Ofsted the powers to inspect academy chains at a systematic level means that too many academies remain exposed to falling standards."