Ofsted has been accused of "cutting and pasting" school inspection reports after two documents contained a number of identical sentences and phrases, it emerged on Friday.
The watchdog is facing criticism after inspection reports which declared two schools inadequate were deemed suspiciously similar.
Union leaders raised concerns inspectors are copying reports, warning there is a problem with formulaic guidelines that inspectors must follow for school visits.
The National Association of Head Teachers (NAHT) said that the practice is unacceptable, given that the fate of a school can rest on an inspection.
Two reports, one on Belvedere Junior School in Bexley, and one on Malmesbury Primary School in Tower Hamlets, east London, were found to include a high number of similarities, the Times Educational Supplement reported.
Ofsted said it stood by the judgments in the reports, but is looking into the similarities in wording.
Both contain the sentence: "Some teachers do not plan learning for pupils at their different levels of ability and marking is not leading to improvement."
And both state: "The majority of parents and carers are positive about how well the school develops their children's skills in reading, writing and mathematics."
In each report, inspectors disagree with this view.
Both also make similar comments about the low attainment of pupils in reading, writing and maths.
One goes on to say: "This is because pupils, especially in Key Stage 2, have not been taught effectively to sound out individual letters and groups of letters to work out unfamiliar words when reading.
"They have not been provided with enough opportunities across the curriculum to write at length and their grasp of basic number skills is not as secure as it should be. These weaknesses mean the curriculum is inadequate.
The second says: "This is because pupils have not been taught effectively to sound out individual letters and groups of letters to work out unfamiliar words.
"They have not been provided with enough opportunities across the curriculum to write at length, and their grasp of basic number skills is not as secure as it should be."
Both schools were visited by a team with the same lead inspector who works for Tribal, a company contracted by Ofsted to carry out inspections.
Belvedere was inspected in January, with the report, which judged the school should be put into special measures, published in March.
Malmesbury was inspected in March, with the report declaring it inadequate and requiring special measures published a month later.
An Ofsted spokeswoman said: "Both inspection reports were subject to moderation by Ofsted and we stand by the judgments they record.
"We are concerned by the similarity of some of the wording used in the two reports and this is being looked into with the inspector concerned."
In a statement Tribal said it aims to ensure that reports "reflect the uniqueness of schools".
"Tribal stands by the judgments in these two particular reports," it said.
"Similarities noted in some of the wording caused concern and are being rigorously investigated. If proven, Tribal will take swift action to stop using the inspector again.
NAHT general secretary Russell Hobby told the TES that he had heard anecdotal accounts of other "cut and paste" reports, adding that this is "not acceptable given that the fate of the school rests on what is written."
NAHT head of research Lesley Gannon said: "Problems are compounded at the second stage of the inspection process when the reports are rewritten by a moderator - someone who has never been to the school - who checks the descriptions to ensure they fit the grades described in the inspection schedule.
"Inspectors just cut and paste, regurgitating wording from the evaluation schedule. Therefore the reports are often more similar than they are individual."
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