School leaders have called for independent research to be carried out into how to ensure Ofsted reports are reliable.
The Accountability Commission led by the NAHT headteachers' union says the inspectorate still has an important role identifying where schools are underperforming and need support.
However, it warns that the current short inspections of schools do not allow Ofsted to make reliable judgements.
It has called for Ofsted to commission research to “determine the format and nature of inspection required in order to provide reliable judgements.”
Nick Brook, the NAHT’s deputy general secretary and chair of the commission, said it is vital that Ofsted reports are seen as reliable in order for it to help identify where schools need to improve.
Ofsted did carry out a study last year sending inspectors to revisit 24 primary schools, and it concluded that short inspections were 92 per cent reliable.
However, Ofsted's own head of research Daniel Muijs told Tes earlier this year it is impossible to determine conclusively how reliable school inspections are.
The new commission report warns there is now a growing consensus in the education sector that short inspections are “of limited reliability because there is so much to get through in a short time”.
It calls for an honest appraisal of what Ofsted can be expected to do after seeing its funding cut back.
The call for research to be carried out to establish how to reliably inspect schools is one of four recommendations the commission has made on Ofsted.
The commission says Ofsted’s work needs to be refocused on identifying school failure and providing more support to help them improve.
It has also called for the removal of an exemption on outstanding schools being routinely inspected. This has led to more than 1,620 schools not being inspected for six years or more.
And the commission has also said that Ofsted’s outstanding grade should be replaced with a new system of identifying excellence.
The report says: “The Commission believes that identification of excellence should be firmly anchored within school improvement rather than used as a measure of accountability.”
It says this should be specific about what a school is doing well and the context in which it is working.
Mr Brook said: “With limited resources, we believe Ofsted should focus attention where it can make the most difference rather than spreading themselves too thinly.
"The report proposes a new role for Ofsted, focusing on identifying failure and providing a stronger diagnostic insight to schools that are struggling. These are the areas where they can have real positive impact.
"Even on short inspections, any Ofsted inspector worth their salt should be able to quickly identify schools which are failing. Likewise, it must be possible for a full inspection of a school to provide a much stronger diagnosis of issues, to enable a better targeting of funded support to actual need."
An Ofsted spokeswoman said: “We welcome any constructive addition to the important debate about our school accountability system.
"That is why our chief inspector, Amanda Spielman, was very happy to take part in the NAHT Commission and will participate in their launch event later this week.
"We recognise that inspection practice cannot stand still, and we believe that Ofsted’s role should complement, rather than magnify, other elements of the accountability system.
"We are currently developing our new education inspection framework and aim to consult on the details in the new year. We look forward to the NAHT’s contribution to that work.”
The NAHT’s School Accountability Commission report is being launched at an event on Friday.