The chief inspector of schools has called for increased powers to scrutinise multi-academy trusts (Mats), warning that parents and policymakers currently have only a partial view of what is happening in England’s schools.

Amanda Spielman, the head of the schools regulator Ofsted, said trusts were not being held to account properly as her inspectors were not allowed to inspect them.

Ofsted’s inspections are limited to a “summary evaluation” based on a sample of schools belonging to a trust, rather than on the trust itself, resulting in a lack of accountability, according to Spielman.

A growing number of schools in England are being taken out of local authority control and turned into academies, which critics have long claimed lack transparency and local accountability.

About three-quarters of secondary schools and a third of primaries are now academies and three-quarters of those belong to a Mat, some of which control as many as 50 schools or more. “Given the power and influence of Mats, it’s important that they are properly accountable to parents,” said Spielman.

“The fact that Ofsted is unable to inspect trusts directly means parents and policymakers are only given a partial view of what is happening in our schools. This presents some very real risks, which we have seen highlighted by the recent failures of some academy trusts.”

The system of summary evaluations of Mats was introduced this year and allows Ofsted to carry out inspections of a number of schools and publish individual reports. Overall findings are discussed with trust leaders before a summary evaluation report on the work of the Mat is published, though an inspection grade, which would be normal with schools, is not given.

Six Mats have been the subject of summary evaluations, among them the Outwood Grange academies trust, which has in the past been criticised for its discipline policy and high levels of exclusions. Ofsted’s report was positive overall but recommended that the trust should reduce exclusions by continuing to improve pupil behaviour.

Ofted published a report on Monday based on an investigation into Mats, which found that schools in larger trusts benefited from economies of scale, back-office support, training, career progression and recruitment. However, it said some Mats had taken on a large number of struggling schools too quickly, without always having the capacity or leadership necessary to improve them.

A Department for Education spokesperson said the academies programme and the freedom it gave school leaders has been at the heart of the government’s education reforms. “Ofsted have already published a number of summary evaluations reports, which are among a wide range of tools we use to hold academy trusts to account. This includes published information about trust performance, annual accounts and letters to trusts where there are issues of under-performance or weaknesses in governance or financial management.”