A new rule on preventative spending will protect budgets whose initial cost is repaid by later savings from being raided for more immediate needs. From the spending review beginning next year, such projects will sit behind a preventative departmental expenditure limit, or PDEL, a fresh piece of Treasury jargon that gives the protection formal footing.

A quiet measure with big ambitions

For a change officials describe as transformative, the announcement was strikingly low-key. It merited the briefest of mentions in John Healey's speech and just one paragraph in the 166-page budget document.

Behind the muted launch sits a large ambition. A government official said departments have too often ended up raiding these types of budgets to fund short-term measures, and framed the policy as an attempt to stop that as part of transforming how the state operates. Officials also tie the decision to a wider focus under Burnham on knotty and neglected issues, including rough sleeping and adult social care.

Some governments already shield individual preventative budgets, such as those for public health, but this is believed to be the first time one has introduced this as a blanket policy.

What could qualify

The budget document gives a concrete example: Andy Burnham's new spending to prevent rough sleeping would qualify for PDEL status. Areas that look likely to fall within the limit include:

• •public health programmes to prevent future illness
• •Sure Start-type early years schemes

The echo of Sure Start is pointed. The austerity measures of the 2010 coalition government slashed numerous prevention-based services, not least the Sure Start programme for families with young children.

The definition problem

What counts as preventative remains unresolved, and departments say they are waiting for Treasury guidance that has not yet been drawn up. That gap matters, because nearly all spending can be argued to fit the label.

Stuart Hoddinott of the Institute for Government said: "A lot depends on the definition ... you could make any argument that almost all types of spending are preventative."

One government official offered an illustration of how far the argument could stretch: the Ministry of Defence could in theory claim all its spending is preventive, given the very obvious costs to the country of fighting a war. The worry among officials is that the policy creates a new variant of the endless spending-review tussles between departments and the Treasury, as ministers work to shield projects from future raids.

Early verdicts

The thinktank had called for such a system to be implemented, and Hoddinott said it could end up being quite a major change in how the government runs spending reviews. Officials in the Treasury agree, with one calling it an important reform.

A cautious welcome came from Greg Fell, who holds both roles: director of public health at Sheffield city council and president of the Association of Directors of Public Health. "This does feel like a really good thing," he said, while flagging questions about how the policy will operate.

Preventing future acute demand, or creating the conditions for communities to thrive - he asked which of the two the policy pursues, noting that different mindsets are needed and some benefits may take decades to appear. He added that as a country we have never been very good at this sort of thing, so it is up to all of us to make sure it succeeds.

For now, the practical test sits with the Treasury, whose guidance on what qualifies has yet to be drawn up.