One of England's biggest academy chains has been condemned by Ofsted after inspectors found that eleven of its schools are failing to give children a good education.
In a damning letter to the E-ACT Trust, Ofsted warned that the chain had failed to take effective action to improve standards in many of its schools.
The schools watchdog had launched a series of inspections of the Trust's schools - visiting 16 of its academies in two weeks amid continuing concerns about under-performance.
Inspectors declared five of the academies inspected - including one new free school, Hartsbrook E-ACT free school in north London - to be failing and require special measures, while a further six were rated as "requires improvement".
Four were judged to be good and just one, Heartlands Academy in Birmingham, was found to be outstanding.
Ofsted said that ten of the academies inspected had failed to improve since their previous inspection, and of these, six had gained a worse Ofsted grade.
In its letter to E-ACT boss David Moran, Ofsted warned that "an overwhelming proportion of pupils attending the E-ACT academies inspected were not receiving a good education."
It added: "The evidence collected during these inspections indicates that intervention and support provided by E-ACT was ineffective overall. For those academies judged to require 'special measures', the Trust failed to take effective action to improve performance."
The letter, signed by Ofsted's regional director for the West Midlands, Lorna Fitzjohn, concluded: "There is an urgent need for E-ACT to take action to tackle underperformance in a relatively large number of academies in the Trust and to ensure that pupils' achievement rises to a good standard."
In a statement, Mr Moran said the inspections had confirmed that standards at some of the Trust's academies were not acceptable.
"No-one should be in any doubt about E-ACT's commitment to the task of improvement. That commitment is driven by E-ACT's mission to break the link between poverty and under-achievement."
He added that E-Act began making changes last May and is making "root and branch reforms" under new leadership.
"Crucially, going forward, we are focusing our resources and support where we are best placed to make a difference," Mr Moran said.
Ofsted said it had seen "poor quality teaching" in many of the 16 E-ACT academies inspected, with the work set not properly matched to pupils' abilities.
It added that lessons were often not challenging enough for bright pupils and there was a "lack of urgency" in taking action to close the gap between disadvantaged youngsters and their classmates.
Inspectors were also told by senior staff at the academies that until the beginning of last September, E-ACT had deducted a proportion of pupil premium funding - money for the poorest students - from each academy.
"It is unclear how these deducted funds were being used to improve outcomes for disadvantaged pupils," Ofsted said.
Ofsted's national director of schools, Michael Cladingbowl said: "The outcomes of these inspections indicate that E-ACT has not been effective in improving its academies. While it is reassuring that some principals of individual academies report recent improvements to the Trust's leadership, inspectors have yet to see this impacting on standards.
"Ofsted is determined to shine a light wherever we have concerns about the quality of education and, where necessary, we will continue to monitor the individual schools within the Trust to ensure progress is being made."
It is the first time Ofsted has conducted a large-scale inspection of schools within the same academy chain.
Schools minister Lord Nash today sent his own warning letter to Mr Moran saying Ofsted's findings were "hugely disappointing".
"We are extremely concerned about the number of your academies that are not yet 'good'," he wrote.
"As Ofsted's letter makes clear, the majority of pupils attending an E-ACT academy are not receiving a good education. This is unacceptable."
Lord Nash said that two E-ACT academies had already been handed pre-warning notices - the first step of formal intervention - and he was issuing a further three.
If improvements are not made, the Education Secretary Michael Gove may look to find new sponsors for more of the Trust's academies, he added.
Ofsted's findings come just weeks after it was revealed that E-ACT is to relinquish control of 10 of its academies amid continuing concerns about under-performance.
In his statement today, Mr Moran said: "The 24 academies that are remaining with E-ACT will be fully engaged with all that we have to offer. A commitment to high standards is worth something only when matched by the will to deliver high standards, and ultimately when those standards become the norm. That's exactly how E-ACT is approaching this new phase."
Shadow education secretary Tristram Hunt said: " By encouraging some academy chains to expand beyond their capacity, Michael Gove has allowed underperformance to go unchecked.
"Now we see a desperate attempt by ministers to try to create the perception that they are on top of things. This is not the case. Michael Gove should give Ofsted the formal powers to inspect academy chains, in the same way he did for local authorities."