Ofsted is “wounded”, “riddled with problems” and not fit to reform itself, according to a union leader.

Mary Bousted, general secretary of the Association of Teachers and Lecturers (ATL), used her speech at the union’s annual conference this afternoon to launch an outspoken attack on the watchdog, which she claimed was “designed to inspire fear and loathing” in schools.

This is just the latest criticism to be levelled at Ofsted, in what has been a bruising few weeks for chief inspector Sir Michael Wilshaw.

Last month, the Policy Exchange think tank called on the watchdog to scrap “unreliable” lesson observations and cut its use of externally-contracted inspectors; in response, Sir Michael unveiled plans to scrap the use of additional inspectors contracted from private firms and scale down inspections for schools rated good.

But, in the most scathing assessment of the inspectorate yet, Dr Bousted (pictured) told the conference in Manchester that Ofsted was a “laughing stock” among teachers, and “so damaged, so tarnished that it has to be radically and completely transformed”.

The high-stakes nature of inspection has created a situation where new school leaders “significantly raise their risk of committing career suicide”, she told delegates.

“And we know one thing absolutely - we cannot leave it to Ofsted to reform itself,” Dr Bousted added. “Frankly, it doesn’t have a clue.  It is clueless, and, because it is wounded, dangerous.”

She mocked the concept of “a new, shiny Ofsted, emerging phoenix-like from the ashes of its former self”.

“Ofsted, for me, is an agency shot through and riddled with problems,” she told reporters ahead of her speech. “The problems of the quality control of its inspection teams will not go away, and that is a cancer eating at the heart of what should be a robust inspection system.”

“The inconsistencies and incoherencies within Ofsted are now too big to be ignored,” she added. “While Ofsted manages its own internal complaints procedure, and while it’s led by a chief inspector who spits blood rather than thinking carefully about the issues and criticisms being levelled at Ofsted, it is not capable of reforming itself.”

Dr Bousted also described Ofsted as a “weapon of terror” that “depresses standards of education in schools”.

“Frankly, the game is up for Ofsted,” the general secretary told ATL delegates. “It is a busted flush. Ofsted can no longer claim that its inspection reports are worth the paper they are written on ... We know that, frankly, it’s a lottery which depends on which Ofsted inspection team turns up - one that has a clue, or one that is clueless.”

She also derided the claim by Mike Cladingbowl, Ofsted’s national director for schools, that it does not grade individual lessons.

“Really?” Dr Bousted asked the conference. “Have I been dreaming all these years?  Have I really not met so many teachers whose professional lives have been ruined, whose professional reputations have been trashed on the basis of 20 minutes’ dodgy observation by Ofsted inspectors?”

She also raised concerns about the time spent by schools preparing for inspections. “How much higher would standards of education be if teachers and school leaders had not been so distracted by the fear and terror that Ofsted brings, and had been able to concentrate their professional energies and focus on what is right for their pupils in their schools?” Dr Bousted asked.

On Monday, delegates passed a motion that Ofsted had lost “almost all credibility with the teaching profession”.

In her keynote speech to conference, Dr Bousted also claimed that, due to the introduction of performance-related pay for teachers, the union was “hearing stories of banks refusing mortgages to teachers because their future pay is so uncertain”.

However, she told journalists that teachers had not in fact had mortgage applications rejected as a result of performance pay, but many were reluctant to agree to a long-term financial commitment “because they have no idea what they are going to be paid”.