What will we learn in the chancellor's spending review?

"It's a big deal for this government," says Simon Case.

"It's the clearest indication yet of what they plan to do between now and the general election, a translation of their manifesto.

"This is where you should expect the chancellor to say, on behalf of the government: 'This is what we're about'."

As the former cabinet secretary, Mr Case was the man in charge of the civil service during the last spending review, in 2021.

On Wednesday, Rachel Reeves will unveil the Labour government's priorities for the next three years. But it's unclear whether it will provide all that much of an answer about what it's really about.

Unlike the Autumn budget, when the chancellor announced her plans on where to tax and borrow to fund overall levels of spending, the spending review will set out exactly how that money is divided up between the different government departments.

Since the start of the process in December those departments have been bidding for their share of the cash - setting out their proposed budgets in a negotiation which looks set to continue right up to the wire.

This review is being conducted in an usual level of detail, with every single line of spending assessed, according to the chancellor, on whether it represents value for money and meets the government's priorities. Budget proposals have been scrutinised by so called "challenge panels" of independent experts.

It's clear that health and defence will be winners in this process given pre-existing commitments to prioritise the NHS - with a boost of up to £30bn expected - and to increase defence spending.

On Sunday morning, the government press release trumpeted an impressive-sounding "£86bn boost" to research and development (R&D), with the Science and Technology Secretary Peter Kyle sent out on the morning media round to celebrate as record levels of investment.

We're told this increased spending on the life sciences, advanced manufacturing and defence will lead to jobs and growth across the country, with every £1 in investment set to lead to a £7 economic return.

But the headline figure is misleading. It's not £86bn in new funding. That £86bn has been calculated by adding together all R&D investment across government for the next three years, which will reach an annual figure of £22.5bn by 2029-30. The figure for this year was already set to be £20.4bn; so while it's a definite uplift, much of that money was already allocated.

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Peter Kyle also highlighted plans for "the most we've ever spent per pupil in our school system".

I understand the schools budget is to be boosted by £4.5bn. Again, this is clearly an uplift - but over a three-year period, that equates to just £1.5bn a year (compared with an existing budget of £63.7bn). It also has to cover the cost of extending free school meals, and the promised uplift in teachers' pay.

In any process of prioritisation there are losers as well as winners.

We already know about planned cuts to the Department of Work and Pensions - but other unprotected departments like the Home Office and the Department of Communities and Local Government are braced for a real spending squeeze.

We've heard dire warnings about austerity 2.0, and the impact that would have on the government's crime and policing priorities, its promises around housing and immigration, and on the budgets for cash-strapped local councils.

The chancellor wants to make it clear to the markets she's sticking to her fiscal rules on balancing the books for day-to-day spending.

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But the decision to loosen the rules around borrowing to fund capital investment have given her greater room to manoeuvre in funding long-term infrastructure projects.

That's why we've seen her travelling around the country this week to promote the £15.6bn she's spending on regional transport projects.

The Treasury team clearly wants to focus on promoting the generosity of these kind of investments, and we'll hear more in the coming days.

But there's a real risk the story of this spending review will be about the departments which have lost out - and the promises which could slip as a result.