Ofsted is an expensive "old banger" being paid for at "Rolls-Royce rates" and should be abolished in its current form, the annual conference of the Association of Teachers and Lecturers (ATL) has heard.
Delegates also heard the increasingly unpopular inspection quango compared to a "capricious monster" that has gone astray and a "fattened calf" ready for slaughter.
Judith Bennett, a member from Oxfordshire, told delegates that the organisation had become "prodigiously expensive" and was not fit for the job.
She said inspectors had been hindered in their work by a restrictive inspection schedule that emphasised safeguarding and attainment.
Ms Bennett said: "(Chief inspector) Christine Gilbert spoke about `Ofsted myths'.
She said a school with low results can be judged outstanding, but we've seen little evidence of that. The myth is her claim that schools with low results can get `outstanding'."
She said Ofsted's attempts to raise the bar were "like moving from the high jump to the pole vault without a pole being supplied".
Ms Bennett said there was a limit to what schools could achieve when pupils' family circumstances could militate against them.
"Home life can create an enormous barrier to success," she added.
Andy Garner, ATL representative for Calderdale, said: "This quango has grown and put its nose into many new areas. Is this a fattened calf ready for slaughter?"
The comments come after two terms of the unpopular new inspection framework.
Schools have complained of an overemphasis on safeguarding and exam results, lacklustre lesson observations and surprise interim inspections.
A recent analysis by The TES found that in the first six months of Ofsted's new inspection regime, the proportion of schools judged "inadequate" almost doubled, while the proportion rated "outstanding" had more than halved.