More than 80 MPs have told Parliament they are "seriously concerned" about reports of Ofsted making "arbitrary" judgments, leading to schools being unfairly marked down and failed.
The politicians, from all three of the main political parties, fear parents and the public are being given a "misleading" picture of school standards by the watchdog's controversial new inspection framework.
They have signed a Parliamentary early-day motion calling on chief inspector Christine Gilbert to revise the guidance given to Ofsted inspectors.
But Ms Gilbert has hit back, telling the MPs their concerns are "largely unfounded" and based partly on "myths", sparking a war of words with teaching union the NUT.
It is the union's fears about the new regime that prompted the motion, put down by former Labour health secretary Frank Dobson.
It said the Commons is "seriously concerned about reports of inspectors making arbitrary judgments based on trivial issues which lead to schools unfairly being marked down or in some cases deemed to have failed".
The motion also voiced concern that "parents and the general public will be given an erroneous and misleading impression of school standards", in turn urging "Her Majesty's Chief Inspector to revise the guidance to schools and inspectors to remedy these concerns".
Figures published last month showed that the first term of Ofsted's new inspection framework had led to the proportion of schools judged "inadequate" more than doubling and those deemed "outstanding" falling by more than half.
The watchdog said this is due to a greater focus on weaker schools as well as a rise in its expectations. Ms Gilbert has written to the NUT to complain about the campaign it is running against the new framework.
She said the union's website is promoting "scare stories" about schools being marked down for "trivial issues" such as a low perimeter fence.
Ms Gilbert said it has been made clear to inspectors that schools should not be failed on issues of this type. She added that the claim from NUT general secretary Christine Blower, that the watchdog was "punishing" schools, was "unfair and inappropriate".
The chief inspector made similar points in her letter to MPs, accusing the NUT of "needlessly worrying its members".
She said no schools had been judged inadequate for minor breaches of health and safety rules and that 90 per cent of heads had described judgments under the new framework as "fair and accurate".
Ms Gilbert wrotes: "I am confident that these myths will lose currency as more schools experience the new inspection arrangements for themselves."
But this week, Ms Blower wrote to Mr Dobson, insisting the framework was "flawed".
"While I recognise that there are many schools which believe that the behaviour and judgments of their inspection teams are fair, the evidence that we have received is that there are many schools where the opposite is the case," her letter said.
"The reality is that schools collectively experience inspections as arbitrary and a little like 'Russian roulette'."