Another academy chain has been heavily criticised by Ofsted after inspectors found that a number of its schools are failing to give pupils a decent education.
The watchdog has written to bosses at School Partnership Trust Academies (SPTA) warning that it has too many under-performing schools which have been in this position for too long.
SPTA is the third chain to be hauled over the coals by Ofsted, with critical letters recently sent to both the Kemnal Academies Trust (TKAT) and the E-ACT Trust - one of England's biggest academy organisations.
Inspectors visited six SPTA academy schools last month and found that four were judged to require improvement. Two had improved from inadequate and the other two had seen no change in their rating.
Of the other two academies inspected, one was still rated as inadequate and the second was judged to be good, up from a previous "satisfactory" judgement under the old ratings system.
Inspectors said they found "key weaknesses" across several of the schools inspected.
These included: teaching that was not consistently good, weaknesses in teaching the brightest pupils and those with special educational needs (SEN), pupil attitudes to learning that were not "consistently positive" and school governance that did not have the expertise to challenge senior staff about shortcomings in teaching and learning.
"Five out of the six academies are not providing a good quality of education," the letter, which was signed by Nick Hudson, Ofsted's regional director for the North East, Yorkshire and Humber, said.
It added: "In one academy, SPTA has failed to tackle significant weaknesses in leadership and management which have declined to inadequate.
"The inspections of SPTA academies since January 2014 show that the percentage of good and better schools is significantly below that seen nationally.
"More positively, it is encouraging that two previously inadequate schools have improved. Also, the percentage of schools showing improvement since the last inspection is higher than found nationally and this gives some cause for optimism."
The letter did say that the overwhelming view of the school principals questioned was that the academies are well supported by SPTA officers, and that the academies value the work of the regional adviser and senior SPTA staff in challenging their performance data and monitoring the quality of teaching.
It concludes that there is some evidence of school improvement, but that the quality and impact of school governing was "variable".
There were further concerns about the "depth and accuracy" of SPTA's analysis of data showing how well pupils were progressing and the contribution this has to improving schools quickly.
"Above all, there are too many underperforming academies which have remained in this position for too long," the letter warned.
SPTA oversees 42 academies in total.
Ofsted chief Sir Michael Wilshaw has been calling for the schools watchdog to be given explicit powers to inspect academy chains, in the same way that they can inspect local council children's services.
In recent months, Ofsted has attempted to overcome the issue by publishing findings of inspections at a number of academies run by academy chains, where there had been concerns about performance.
Earlier this month, the inspectorate warned that an "overwhelming" proportion of pupils attending some of the Kemnal Academies Trust (TKAT) schools are not getting a decent education.
In a highly critical letter to the trust, the watchdog said that while the trust was getting better at supporting its schools, in many cases this had come too late.
TKAT said it was "committed to educational excellence" and is working with its schools to make changes.
Inspectors visited six TKAT primary academies over a two-week period amid wider concerns that standards in the Trust's schools were not up to scratch.
And earlier this year, Ofsted sent a similar letter to the E-ACT Trust , warning that it had failed to take effective action to improve standards in many of its schools.
E-ACT said it has been making changes for over a year and introducing reforms under new leadership.
In a statement, SPTA said: "The SPTA recognises the important role that Ofsted plays in monitoring standards in the school system. The trust also recognises that Ofsted comments around areas for improvement are suggested on the basis of constructive dialogue to ensure all children receive a first class education, regardless of the school setting.
"Equally however, the trust also recognises that the evidence to support these comments was largely drawn from a small sample of six schools, in a multi-academy trust that supports in excess of 42 schools and which contains two teaching schools, accredited through the National College of School Leadership.
"Whilst the trust looks forward to discussions with Ofsted about how to improve our performance, it is important to consider the facts in relation to the whole group, not just the six schools that were inspected."
The trust said that overall, 13 of its primary academies (48%) are rated as good or better, along with five (33%) of its secondary academies.
It added that during the 2013/14 academic year, Ofsted inspected nine SPTA primary academies and four of its secondaries. Ten of the schools have been sponsored for under two years.
Of the schools inspected, four primaries moved from "requires improvement" to "good", two went from special measures to "requires improvement", three primaries and three secondaries maintained their previous rating and one secondary was graded as outstanding in all areas.
"Whilst the trust acknowledges the need to improve our schools further for young people, given the inherited Ofsted profile on conversion, it is not surprising that any analysis of the current number of good or better schools in the group would be concerning for Ofsted," SPTA said.
"However, there is ample evidence, as outlined above, which when compared to national trends and statistics, confirms the board's belief that SPTA is a highly effective sponsor and that through Teaching School status, there is national recognition that SPTA is making a valued contribution to the overall school system."