Kevin Courtney, the deputy general secretary of the National Union of Teachers has questioned why these grades, which the Department for Education gives to academy trusts, are not made publicly available.
His comments follow Education Secretary Nicky Morgan writing to the chief inspector for schools Sir Michael Wilshaw to make clear that Ofsted can look at the work of academy chains when it inspects schools.
Academies are state schools which are funded directly from central Government.
Increasing numbers of these academies are run by chains which are responsible for a group of state schools.
Mrs Morgan has said Ofsted should be able to publish information about the performance of multi-academy trusts (MATs). But she said it must not give an overall “binary” judgment about whether a trust is effective or not.
Sir Michael has previously said he believed Ofsted should be able to inspect academy chains.
Now Mr Courtney is questioning why the DfE will not allow its own separate assessment of academy sponsors to be published.
He said: “It now appears that the DfE is happy for Ofsted to publish information about the performance of academy chains.
“While we welcome this it throws into sharp focus the fact that the department itself already assesses the performance of academy sponsors and produces ratings for them but is not willing to share this information with the public. Why not?” Earlier this month Labour’s shadow education minister Kevin Brennan asked in a written Parliamentary question if these academy sponsor ratings would be published.
This was rejected in a written response by minister Edward Timpson. He said: “The disclosure of this information would prejudice, or would be likely to prejudice, the effective conduct of public affairs.”
The DfE has also refused to disclose the ratings following a freedom of information act request from journalist and former teacher Laura McInerney. Last year she asked the department to produce “a list of academy trusts and sponsors and their internally allocated grades or ratings.”
She also asked for internal guidance documents explaining how these ratings are decided.
In response the department confirmed that it did hold the information being asked for. However it said it would not be in the public interest to publish.
The decision letter added: “Sponsor grades are subject to change and therefore the release of this potentially misleading information would not be in the public interest.”
The DfE also argued that releasing the individual grade of a lower rated academy sponsor might reduce the likelihood of it applying to run a school where it would be well suited “as a result of believing that there was little point in being considered.”
The issue of Ofsted inspecting academy chains had been raised last year at Education Select Committee hearings.
Mrs Morgan had told MPs that inspectors already had sufficient powers to look at the work of chains when it looked at their individual schools. She dismissed the idea that Ofsted needed new powers to directly inspect academy chains.
At the time committee chairman, Beverley and Holderness MP Graham Stuart said the decision was bizarre.
He said Ofsted needed to be able to look at what he described as the “control centre” of schools.