Since Sir Michael Wilshaw took up the reins at Ofsted in 2012, the inspectorate has been in a state of near-permanent revolution. The “satisfactory” grade has been scrapped, graded lesson observations - long feared by teachers - appear to be on the way out and learners can now rate their college online.

And although the announcement that contracts outsourcing inspections will end next year may appear less significant for the average classroom teacher, for Ofsted this could be the biggest challenge of all.

Since 2009, three firms - CfBT, Serco and Tribal - have been responsible for providing the vast majority of inspectors. Ofsted directly employs just 300 Her Majesty’s Inspectors (HMIs), long regarded as the most trusted guardians of the inspection regime, and external companies provide the 2,000 additional inspectors (AIs) required to make the system work.

When the six-year contracts were signed, Christine Gilbert, then chief inspector, confidently proclaimed that they would “ensure Ofsted has a high-quality, flexible and diverse workforce helping deliver consistency and value for money”.

The “value for money” aspect was key. Ofsted’s funding has been slashed over the past decade, and outsourcing responsibility for the bulk of its inspection team was a crucial method of cost-cutting. Today, the contracts are worth pound;40 million a year, equating to a quarter of Ofsted’s budget.

But the question of whether the changes have resulted in the development of a high-quality bank of inspectors, who are capable of accurate judgements, is problematic for Ofsted.

The inspectorate’s official line is that the system has delivered a “successful and professional inspection programme”. CfBT chief executive Steve Munby went even further, insisting that his company had “developed a workforce at the top of its game”. But the regime has come under fire at both ends of the political spectrum, and teachers have repeatedly expressed concerns about inconsistent judgements and poorly prepared inspectors.

“We want to be confident about any inspection team, that inspectors have a relevant background that allows them to pass judgements,” said Christine Blower, general secretary of the NUT teaching union. “But people don’t have any confidence in them.”

Right-leaning thinktank Policy Exchange also has serious reservations about whether recruits have the right skills and are properly trained. In a report published in March, it says that many inspectors have not worked in the type of institution they are inspecting, nor are they required to have recent experience at the chalkface.

Pay, too, is “not high enough to attract the calibre of professional required”, the report argues, adding: “The majority of teachers and headteachers spoken to for this research held HMIs in much higher esteem than AIs.”

The report also raises fears about the difficulty of passing on new directives from Ofsted to externally contracted inspectors, which it says can result in “Chinese whispers”, with schools all too often bearing the brunt of the confusion.

On this point, it seems that Sir Michael agrees. “Inspection is just too important for Ofsted to simply have oversight of third-party arrangements,” he said earlier this year.

A source who has worked for one of the providers told TES that it had recently cut its pay for AIs to around pound;300 a day. “It has made the role unattractive to many in the profession,” he said. “They always need more people, so the threshold [for new recruits] is low.”

Financial pressures had also led to an increase in “copy and paste” inspection reports, he added. “Inspectors don’t have time to do the work. They rush into schools, having been given the data the day before, make their assumptions, watch a few lessons which confirm their prejudices, then go away and write it up before moving on to the next inspection.”

In 2012, after enquiries by Stephen Ball, principal of New Charter Academy in Greater Manchester, the inspectorate was forced to admit that it did not know how many HMIs had led outstanding schools. Later that year, Tribal admitted to TES that it employed at least five AIs who were not qualified teachers.

In 2013, Ofsted agreed to publish “pen portraits” on its website detailing the background of each of its inspectors, although these were subsequently criticised for being “exceptionally bland” by Policy Exchange.

Lorna Fitzjohn, Ofsted’s national director of further education and skills, told TES that no longer using outsourced inspectors would give the organisation more “control”.

“I don’t think we’re looking at something that’s particularly broken,” she said, adding that bringing all inspection in-house would lead to “more control, more flexibility around the workforce and more opportunity to do more training with them”.

Ms Fitzjohn was adamant that the change would not be a strain on Ofsted’s finances. “It can’t cost more money,” she said. “Like all government departments we have restrictions on budgets, so, no, we wouldn’t expect it to cost more. I think realistically, though, it’s not going to save us money.”

So will the changes bring an end to accusations of inconsistency against inspectors?

Mr Ball is not convinced. “At last Ofsted have taken note of the massive inconsistency and variation in the quality of AIs,” he said. “It’s something I very much welcome. However, Ofsted has a big job to do. The leadership are going to have to gear up for a lot of quality assurance work they’re just not used to doing.”