Over the last two decades, while English schools have been subjected to a permanent revolution of government initiatives, one institution has remained unchallenged. Ofsted – the Office of Standards in Education – has been the accepted guarantor of quality.

For politicians, press and public alike, the investigations and reports of the Ofsted inspectors have been key to understanding how schools have performed. Until this year, no politician would have dreamed of removing Ofsted inspectors from schools.

Ofsted's reputation rests on its success in replacing the discredited inspection systems of the 1970s.

The scandal at William Tyndale school in Islington where children appeared to be learning nothing and the local authority seemed to be unaware of the failings in the school, brought matters to a head.

By the time Labour prime minister James Callaghan made his famous Ruskin College speech in 1976, calling for reform, he did not even bother to tell the chief inspector he was going to make it. What had become known as the 'secret garden' was about to be subject to regular and thoroughly independent inspection, and Ofsted was the result.

Ofsted had its roots in the belief that schools were walled off from investigation and accountability. The term 'secret garden' had been coined in 1960 by Tory education secretary Sir David Eccles to express his concern that he did not know what happened in schools.

However neither he nor his immediate successors wanted direct intervention. Callaghan's Ruskin speech changed that. The door into the 'garden' was opened with subsequent governments actively intervening. The creation of Ofsted was the logical outcome.

Teachers have never favoured Ofsted, but their criticisms have fallen on stony ground. Indeed despite reductions in the number of inspectors, which left those remaining stretched dangerously thin and challenged by having to take over inspecting social services, Ofsted has never seriously lost credibility. It has become an essential tool to maintain public confidence in state education.

It therefore came as a shock when education secretary Michael Gove proposed to exempt categories of schools from inspection. This is one of the 50 extra powers he is taking in the current education bill, without much debate.

The key issue is section 39, under the misleading heading 'Standards' which refers to 'School Inspections: exempt schools'. This needs immediate attention.

At first sight, the changes seem minor since inspections of a kind will remain. What will remain is the old Victorian system of HMI inspection, allowed under section 8 of the 2005 Education Act. But these have always been few and far between. It is the regular Ofsted inspections under section 5 of the 2005 Act which matter, and on these the bill amends the 2005 Act in two crucial ways. Firstly, Section (4A) states that "Regulations may provide that this section does not apply to prescribed categories of school in prescribed circumstances". This is glossed by section (4B) which states "a school to which this section does not apply by virtue of regulations under sub-section (4A) is an 'exempt school'".

The regulations are not specified, nor the categories of schools to which the exemption will apply. Indeed, the circumstances which would lead to schools being removed from the inspection regime are obscure. All that is clear is that the secretary of state will make these decisions. But the question has to be asked, is it in the public interest for any institutions to be exempt, thus returning them to the secret garden?

It is currently suggested that Grade 1 schools as approved by Ofsted might become exempt from inspection. But no school however good comes with a guarantee of permanent success. Standards can and do slip. In its briefing to the Lords, Ofsted revealed that 31% of schools graded outstanding in an inspection do not maintain this standard in the next inspection. Given that top grading allows inspections to be postponed, it is clear that nearly one third of outstanding schools take their foot off the gas when the regular inspection is not imminent. How much worse will the situation become if they have no regular inspection at all?

Ofsted is planning a 'metric' system, where data on schools will be checked to trigger an inspection if problems are indicated. But data can be massaged, and in any case will only be gathered three years after a school inspection. If problems are indicated, HMI will conduct a desk check of the data. Only if they then decide that there is a case for inspection will Ofsted actually go into a school. By that point, a school with problems would have damaged the education of a large number of children. Metrics are not acceptable.

The public has a right to know whether standards are being maintained or not. If the inspection system is to retain credibility, regular inspections are vital. The government plans to transfer money, curriculum control, admissions and strategy to school level. Surely the government would welcome independent confirmation that public resources are being used well?

The inspectors will be barred from regular visits if this section of the bill becomes law with Section 39 in place. Selected categories of schools will become exempt, and these schools will be selected solely by the secretary of state. This is unacceptable. The regular inspections by Ofsted under section 5 of the 2005 Act must continue. Parliament must throw out section 39 of the current education bill. There must be no return to the secret garden.

•Trevor Fisher is editor of Education Politics and a retired history

lecturer. Lord Hunt is shadow deputy leader of the opposition in the House of Lords and a keen supporter of comprehensive education.