Ofsted inspectors have accused one of the biggest academy chains in England of "low expectations".
Inspectors visited 12 schools run by the Academies Enterprise Trust (AET) and found too many pupils were "not receiving a good enough education".
A survey of the group's head teachers found doubts about the ability of the trust to support their schools.
A statement from AET said the findings did not give "the true picture" of the full range of the trust's 77 schools.
Ofsted has published the findings of a wave of inspections of academies operated by AET, following concerns they were not improving quickly enough and too many were "less than good".
Inspectors are not allowed to assess the trusts running academies, but they can inspect their individual schools - and in this case, 12 were inspected, in areas including Essex, Liverpool, Birmingham and Hull.
'Left to flounder'
Ofsted found:
None were outstanding
Six were good
Five required improvement
One was inadequate
Inspectors highlighted a lack of overall progress and identified "key weaknesses", including teaching not being good enough.
The inspection findings warned of "low expectations of what pupils can and should achieve" and "pupils with less than good attitudes to learning and unacceptable behaviour".
Ofsted said some schools were "left to flounder".
There was also a survey of school leaders working in AET academies.
It found "academy leaders did not have confidence in the trust's ability to provide the support they needed".
Earlier this year, the Department for Education included AET on a list of academy sponsors restricted from taking over new schools.
The recently appointed chair of Ofsted, David Hoare, is a former trustee of AET.
'Unfairly negative'
A statement from AET said that across its full range of schools exam results were improving faster than average.
The trust also challenges the "unfairly negative slant" of the inspection report summaries.
"We have raised a number of issues with Ofsted about their interpretation of the data and potential errors of fact," said the trust statement.
"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."
Sir Michael Wilshaw, head of Ofsted, has called on the government to allow inspectors to be able to check academy trusts, as they can for local authority education services.
A majority of secondary schools in England are now academies, which are state-funded schools outside of local authority control and with flexibility over areas such as the curriculum.
Labour's shadow education secretary Tristram Hunt said there was inadequate oversight of academies and "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," said Mr Hunt.