The occupational pensions of army generals and top Whitehall mandarins will be classified as welfare spending in the tax transparency statements that George Osborne has promised every taxpayer.

Her Majesty’s Revenue and Customs is writing to millions of tax-paying households with detailed figures on how the government spends their income tax and National Insurance contributions. Welfare is recorded collectively as the single largest expenditure, consuming nearly one pound in every four.

This presentation has been criticised as a politically motivated departure from Treasury officials’ original plan to break down social security into the components paid to different parts of the population, such as elderly, disabled and unemployed people.

By revealing that payments specifically earmarked for the unemployed, for example, represented only 3% of the total, this approach may have set back Osborne’s case for a fresh £12bn in benefit cuts.

Now experts are drawing attention not only to the lack of differentiation in the welfare chunk of spending but also to the inclusion of substantial elements of spending that would not normally be considered welfare, notably personal social services and public sector pensions.

The Treasury said: “The headings in our tax summaries are based on internationally recognised (UN) definitions.” But in a briefing note published on Tuesday, the Institute for Fiscal Studies detailed how the welfare total included £28.5bn of “personal social services”.

“This is a number that in many analyses one would want to report separately from other welfare spending,” the IFS said. “Unlike other elements of ‘social protection’ it is not a cash transfer payment and in many ways has more in common with spending on health than spending on social security benefits.

“Another £20bn of the spending counted under welfare is pensions to older people other than state pensions. That includes spending on public sector pensions – to retired nurses, soldiers and so on. This is not spending that would normally be classed as welfare.”

Declan Gaffney, a social security researcher, said the inclusion of public sector pensions was bizarre. “The Treasury needs to clarify exactly how it arrived at these figures, and publish the workings – spelling out exactly whose pensions it included and why.” After a day of confusion, in which the Cabinet Office originally led the Daily Mirror to believe that MPs’ and ministers’ pensions would be classed as welfare, the Treasury belatedly clarified that, because the parliamentary pension scheme is funded, it would not be included with the unfunded scheme which covers their civil servants.

Gaffney has used IFS tables to calculate a more conventional figure for total welfare less state pension expenditure, and concludes that the government’s choice of definition inflates the published welfare spending total by around 40%.

A spokesman for PCS, the civil service union, said: “Tens of thousands of civil servants work hard to deliver social security support and they know how important and necessary it is. For their pensions to be hijacked as part of the government’s latest political attack on our welfare state is absolutely disgusting and it exposes just how far ministers will go to poison the well of public opinion.”