Are Reeves’s budget plans already making investors take fright? Or is that a market misreading?

Budgets are all about numbers.

In the coming 24 hours, we’ll be engulfed with all sorts of figures – about the state of the economy, about the size of the deficit, about the fiscal rules the new chancellor is planning to introduce in the coming months.

But in fact most budgets, this one included, can really be boiled down to the difference between two big numbers.

Politics live blog: Budget 2024 latest developments

Total government spending and total government receipts.

Right now the UK government is spending just over £1.2trn a year and bringing in just over £1.1trn in taxes and receipts.

In other words, this country is spending more than it generates in tax receipts.

So it has to borrow the difference.

That borrowing, also known as the deficit, is (as you’ve already probably worked out from the above numbers) around £100bn a year.

And politicians, including the chancellor, spend rather a lot of time fretting about the deficit.

Indeed, the main objective of the various different fiscal rules they’ve imposed on themselves in recent decades has been to narrow the gap between those two big numbers.

Broadly speaking, the easiest way to do this is to cut something few people notice in the short run – government investment.

When he came into office in 2010, George Osborne cut a lot of parts of public spending, but he absolutely slashed the amount the public sector spent on buildings, infrastructure and machinery – capital spending.

Having lifted the total briefly after the pandemic, Jeremy Hunt was planning a similar fall in investment in the coming years.

Rachel Reeves has said repeatedly ahead of the budget that she plans to invest far more in the coming years.

This is a noble goal, given investment tends to benefit future generations, however, it will not be cheap in the short run.

Indeed, keeping investment spending at current levels will cost roughly £30bn a year by the end of this decade.

So how does the chancellor square that with her fiscal rules?

Well, one part of the answer is that she’s planning to increase the revenues coming into the Exchequer, reportedly via higher national insurance charges for insurers.

But the other part of the answer is that she’s changing her fiscal rules as well.

Budget 2024: Rachel Reeves vs the fiscal rules

The long and the short of it is that Ms Reeves looks likely to choose a set of fiscal rules that ignore investment spending.

Both her updated debt rule and her current budget rule essentially omit capital spending – although they include debt interest costs, so she can’t just borrow willy-nilly.

That might sound like fiscal jiggery-pokery, and some in the market fret that investors will soon take fright as a result.

Indeed, some suggest they already are, and point to the fact the UK’s cost of government borrowing – as measured by the benchmark 10-year bond yield – has risen from under 4% to nearly 4.3% in the past month alone.

However, this is a slight misreading of this market, which is as affected by global economic factors and central bank action as much as by UK budgetary policy.

Indeed, compare the recent changes in the UK’s borrowing rates with those in Germany and the US and British government bond yields are close to where they usually trade in the run up to a budget.

And they are far, far below where they were in the run-up to Liz Truss’s mini-budget.

Even so, there are bound to be a few unexpected surprises and some relevant new data points in this fiscal event.

It is a budget after all.