The government has assessed the amount of money it will save through its controversial "stealth" cut in school funding for the country's poorest pupils, Tes can reveal.
But the Department for Education is refusing to release the result because of a fear that doing so could "harm" its "reputation".
Critics described its position today as "downright deceitful".
The DfE had already been condemned for its refusal to say the amount that schools are set to lose as a result of its decision to calculate pupil premium allocations for 2021-22 based on a census from last October, and not in January – when more pupils would have been eligible – as had been expected.
But the news that it has already worked the total out but still won't release it has added to the controversy.
Investigation: 'Come clean' on funding gap, DfE told
Kate Green, Labour's shadow education secretary, has described the policy shift as a "stealth cut", arguing that the Conservative Party has "neglected children through this pandemic".
Despite repeated questioning from Tes, the DfE has so far refused to quantify the impact on school budgets as a whole.
It previously said the information was dependent on census data – and that figures for January 2021 would be publicly available in June.
DfE 'deceitful' over pupil premium funding
Now Tes has learned that the DfE already has an estimate for the financial impact of the change, but still won't say what it is.
In response to a freedom of information (FOI) request from free school meals campaigner Andy Jolley, seen by Tes, the department said it holds information "regarding an assessment of the financial impact of the change in pupil premium collection date", but this is being "withheld".
The DfE said arguments in favour of disclosing the information would include that "there is a public interest in understanding the financial impact of the switch to using the October census", and that "releasing the numbers would contribute to the transparency of government business".
However, it said making the data public "could give rise to a misinterpretation of information" – citing fears over the impact on its own "reputation".
"The estimate we hold carries a very large uncertainty due to the unknown impact of the pandemic on the number of pupils attracting free school meals," the department said.
"If the estimate turns out to be significantly different from outcomes, it could harm the department's reputation in regards to the accuracy and credibility of the statistical information it produces."
Wes Streeting, Labour's shadow schools minister, also requested a copy of the financial assessment in a written question to the DfE.
However, schools minister Nick Gibb failed to acknowledge the existence of the estimate in his answer, published yesterday.
"The department will confirm pupil premium allocations for the financial year 2021-22 in June 2021," he wrote.
"This will provide the public with information on the specific amounts that regions, local authorities and schools are receiving through the pupil premium for 2021-22."
Mr Streeting told Tes that the government had "fiddled the census dates used to calculate pupil premium, with a view to deliberately shortchange schools of millions of pounds".
"Not only is this a total disgrace, their refusal to come clean with the financial impact assessment, showing how much schools have been shortchanged by, is downright deceitful," he said.
Mr Streeting also said that Mr Gibb "obfuscated" in answer to his question, adding: "When you ask very specific questions for information, and you end up getting a broad generalised answer that doesn't actually address your question, I think it's just an insult to Parliament and to the public."
In its response to Mr Jolley's FOI request, the DfE went on to say: "Analysts in the department produce a range of forecasts and estimates. They have robust methods and processes in place for analysing and quality-assuring assessments – and it is important that the public maintains trust in the statistical information released by the department.
"For internal purposes, analysts sometimes undertake much less robust calculations, as new ideas are developed and options discussed. This can also be done to test new approaches and forecast methodologies.
"It is important for effective conduct of public affairs, in particular for evidence-based policy-making, that policy leads can commission analysts to undertake a range of analyses, and analysts themselves can maintain a safe space to develop their analysis in this way, without worrying that emerging work and ideas, which may unavoidably be based on less robust calculations, and/or have had less stringent quality assurance applied, will later be released for publication.
"It could stifle the incentive to use data and associated analysis to inform policy decisions if all such work could be made public later on."
The DfE told Tes that it moved to using the October census to calculate pupil premium allocations so that all schools could know their budgets earlier in the year, helping them to plan ahead.
Data on the number of pupils eligible for the funding in 2021-22 will be published in June, the department said.
It added that the number of pupils that would have been eligible for the premium if the census date had not been changed was not yet available.