Around 780,000 pensioners in England and Wales are set to lose their winter fuel allowance because they are not expected to apply for benefits they are entitled to, according to the government's own analysis.
The Department for Work and Pensions (DWP) estimates were released under freedom of information laws on Friday.
The "equalities analyses" predicted many people entitled to pension credit - and therefore eligible for fuel payments under the new rules - will not submit a claim, and therefore lose the subsidy altogether.
The disclosure comes after the government told opposition parties it had not carried out a full assessment of the policy's impact, amid calls for analysis to be released.
No 10 has previously said it was not legally obliged to produce a full impact assessment on the decision to means test the benefit.
Under the changes, winter fuel allowance will still be available to those claiming pension credit and some other benefits, but around 10 million people are set to be stripped of the payment.
A Treasury spokesman told the BBC that more than a million pensioners would still receive the winter fuel payment, and the government is encouraging people eligible for pension credit to apply.
However, the DWP's internal analysis confirms the government is expecting hundreds of thousands of eligible people to miss out.
The government estimates around 100,000 more people could be persuaded to claim pension credit, but more than three quarters of a million pensioners who are eligible are still not expected to submit a claim, the document confirmed.
The analysis also suggested that nine in 10 pensioners aged between 66 and 79 would lose their allowance, and eight in 10 over 80s would do so.
Since those over 80 receive a higher payment - £300 as opposed to £200 - they would take the greatest financial hit, it said.
The analysis also found that while those with a disability would be most likely to retain the payment, around 71% will still lose their entitlement.
Conservative Party chairman Richard Fuller called on Labour to "immediately" conduct and publish a full impact assessment of "this harmful policy", accusing the government of having "sneaked out" the analysis.
Previously, the Liberal Democrats said not carrying out an impact assessment before cutting support was "absolutely unthinkable".
Speaking on Thursday, Sir Keir Starmer said the decision to cut winter fuel payments was necessary to stabilise the economy, and that the government was putting "mitigations in place".
The DWP said the document - which is more limited than a full impact assessment - was not "routinely published alongside secondary legislation" after it was published in response to a Freedom of Information request.