Education Secretary Ruth Kelly is under pressure to overturn an inspection which failed Islington Green school seven years ago.
The TES reported last week that papers released under the new Freedom of Information Act showed that in 1997 Chris Woodhead, then chief inspector, overruled senior HMI inspectors to place the school in special measures.
The letter from a member of the HMI team showed that the group unanimously felt the school was not failing and that they were "under pressure" to say it was.
Tony Garwood, head of Islington Green from 1996 to 1998, has written to Ruth Kelly, the Education Secretary, and to the Prime Minister urging them to overturn the inspection verdict retrospectively.
He said that the consequences of placing the school in special measures had been catastrophic for staff and pupils.
"Many colleagues who worked at Islington Green have suffered in their careers because of this unjust decision," he said.
"However, the greatest victims have been the children of Islington. At least pupils will now know that they did not go to a rubbish school and their teachers were not incompetent."
Mr Garwood said that there had been speculation that there was a political motive behind the decision to fail the school. "It may have been that letting an Islington comprehensive off the hook within weeks of a Labour election victory was incompatible with the tough line the new Government wished to convey," he said.
Ken Muller, the National Union of Teachers' representative at Islington Green who obtained the HMI letter, has also written to Ms Kelly calling for the 1997 inspection report to be annulled.
The NUT said it suspected that teachers at other schools would use the Freedom of Information Act to find out about inspections.
Mr Woodhead said he had been within his rights to disagree with the HMI team and side with Islington Green's original inspectors, who failed the school.
The Office for Standards in Education said that David Bell, the present chief inspector, had endorsed all but five HMI judgements last year.
The Department for Education and Skills has denied any conspiracy to close Islington Green or that the Government influenced Ofsted's judgements.
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