A former Ofsted inspector has told Sky News the watchdog's process for grading schools is "nonsense", claiming evidence is made to fit a predetermined rating.

The education regulator has come under scrutiny following the suicide of headteacher Ruth Perry, who led Caversham Primary School in Reading, Berkshire.

The headteacher took her own life in January while waiting for an inspection report that was about to downgrade her school from "outstanding" to "inadequate".

So far Ofsted has resisted calls from unions to scrap the single-word judgements, but one former inspector, Andrew Morrish, told Sky News it is time for a "complete reset".

"Reducing it to one single grade is nonsense," he said. "You may as well just toss a coin in terms of getting it accurate.

Mr Morrish estimates he inspected up to 60 schools between 2012 and 2015, before becoming disillusioned with the job and quitting.

He said he would be part of a small team of inspectors, but the process was driven by the "confirmation bias" of the team leader and the need to find evidence to "fit the narrative".

The former headteacher said he could recall a number of times when he was told he "might want to go and rethink" what he put in an evidence form, as it might not have reconciled with what his colleagues were seeing.

He added: "Invariably it's a case of 'well, go and find something else, and keep looking and keep looking and keep looking' until we find some evidence that fits the narrative of what this school is most likely looking to be in terms of that one-word judgement."

According to research by the University of Leeds, stress caused by Ofsted inspections has been cited by coroners as a factor in the deaths of at least 10 teachers since 1998.

And many in the sector fear there may be more if nothing changes.

One union, the National Association of Head Teachers (NAHT) has called on schools to remove any mention of Ofsted from their school buildings and websites.

Its general secretary, Paul Whiteman, told Sky News that distress over Ofsted inspections is "not a new phenomenon", adding: "In 2018, we produced a report about inspections and the pressures, but obviously with the tragedy with Ruth Perry, they have been receiving calls of despair and feelings of not being able to sleep at night because of the absolute worry."

He continued: "People might not know that you can get a telephone call telling you you are going to be inspected on a Monday, Tuesday or a Wednesday… so school leaders probably don't sleep on Monday night, Tuesday night, and then they might be able to relax on Wednesday afternoon when they know that a call is not going to come in.

"That's how impactful waiting for that call is, every week."

Headteacher and NAHT president-elect Simon Kidwell's school in Cheshire was inspected last month.

Despite it being his eighth inspection as a head, the stress of it still triggered his insomnia.

"I think it's the intensity of the process. I think it's the adrenaline because it's very, very high stakes. And it's worrying about your staff," he said.

"I'm getting members of staff saying I'm not sure I can go through another inspection cycle."

Mr Kidwell, described the Ofsted process his school undertook as "very intense" after they had been expecting a phone call from the regulator for 18 months.

"We had four inspectors on site for two days and they were very, very thorough in the things that they looked at," he said.

He said he feels a "huge sense of relief" now that the inspection is over.

"Now we know we are going to get a period of time without Ofsted coming back - we can go to London in June with the pupils and not worry about Ofsted phoning."

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He said there are high stakes involved, with schools having to be prepared at every moment to mobilise their staff and get the right individuals in front of the inspection team if they receive a call.

In a statement, Ofsted said: "Our inspections are first and foremost for children and their parents - looking in depth at the quality of education, behaviour, and how well and safely schools are run. We always want inspections to be constructive and collaborative and in the vast majority of cases school leaders agree that they are."

Ofsted chief inspector Amanda Spielman previously said she has no "reason to doubt" the inspection before the death of Ms Perry and that the "findings were secure".

She has also defended Ofsted's one-word assessments, which have been criticised for being too simplistic, arguing they are easier for parents to understand.