Ofsted has been dealt a major new blow by a heavyweight commission that claims it cannot make reliable judgements about schools and is giving false assurance to parents.
Tes can reveal that a damning new report on school accountability warns that Ofsted does not benefit the vast majority of schools it inspects and cannot provide the level of assurance it is expected to deliver.
The commission questions how much Ofsted can discover in one- or two-day inspections.
However, Ofsted has insisted that its current shorter inspection model does allow it to provide assurance about whether schools are still good or not.
The criticism from the commission is the latest setback to hit the inspectorate after it was attacked by the Public Accounts Committee last week.
The committee criticised Ofsted' s performance with chair Meg Hillier MP warning that Ofsted's credibility could evaporate if school inspection is eroded any further.
And MPs called on chief inspector Amanda Spielman to speak out more after being disappointed with the answers she gave to the committee about the impact of funding pressures on the standard of education.
The chief inspector has insisted that Ofsted had not seen evidence that funding pressures were harming the quality of education in English schools.
But this sparked calls for the inspectorate to be abolished from Kevin Courtney, the joint general secretary of the NEU teaching union, who claimed Ofsted is unable "to speak truth to power."
This wave of scrutiny of Ofsted's work comes after rising tensions between the Department for Education and the inspectorate about its new plans for inspection framework.
Education secretary Damian Hinds has repeatedly failed to back Ofsted's plans for a new inspection framework and raised concerns that it might increase teachers' workload.
Now Tes can reveal that the NAHT heads' union's Accountability Commission has concluded that Ofsted cannot deliver reliable judgements because of the lack of time and resources it has available to inspect schools.
The commission, which includes experts from across the education sector, was set up earlier this year by the headteachers union to identify ways of improving the country's high-stakes accountability system.
Its new report, Improving School Accountability, seen by Tes, highlights a series of concerns about the inspectorate.
It warns that Ofsted is now at a crossroads and called for an honest appraisal of what the inspection body should be expected to do on a massively reduced budget.
The findings follow a critical National Audit Office report into Ofsted, which warned that shorter and less frequent inspections had reduced the inspectorate's capacity.
Ofsted now inspects schools for either one or two days after seeing its budget cut by more than £100 million in just over a decade.
Luke Tryl, Ofsted's director of corporate strategy said: "We are confident that our inspections provide parents, schools and the government with the assurance they need that schools remain good, and that we do so in a way that compares very favourably in terms of quality and value for money with school inspection regimes internationally.”
The NAO warned that shorter inspections give less assurance and provide inspectors less time to discuss with schools how they could improve.
The new accountability commission has echoed these concerns and also highlights other issues facing Ofsted.
It warned that school inspection leads to extra workload which takes time away from teaching and learning and is creating a “culture of compliance” which is holding back school improvement.
Nick Brook, the NAHT's deputy general secretary who chaired the commission said: "We have heard from members who tell us that while they have been impressed with Ofsted inspectors they are literally dashing around the school like headless chickens because they just have so much to get through. They are having to form judgements on the move.
"And we have heard from members who are doing inspection work who describe it as an impossible job. It does not give us confidence that this is a system that can produce reliable judgements.
The accountability report says: “Ofsted no longer has the capacity to inspect schools in any real depth. Even a full inspection only lasts two days and this means inspectors have to make significant, complex judgements about a school in a very short space of time with limited evidence to draw on.
"It is harder to make reliable and valid judgements about the quality of teaching in a school when often inspectors only have a matter of a few hours to do so.”
However, the report does find that school inspection still provides an important function to the education system by identifying schools that are failing and prompting action to improve standards.
And it credits Ofsted with having played a significant role in driving improvement over the past 25 years.
However, it says there is “little evidence to show a positive impact from inspection on school improvement in the vast majority of schools that are not failing.”
Mr Brook said there needed to be a realistic appraisal of what Ofsted is expected to deliver and how it should be funded.
He said: "The commission’s view is that there is a clear choice to be made. Either there must be a significant investment to ensure reliable and insightful inspection for all or a revision of what can reasonably be expected from inspection."