Ben Zaranko, economist at the Institute for Fiscal Studies (IFS), suspects there are several causes.
These include a rise in the number of public sector workers – there are now almost 6m people employed by the state, the highest number since 2012 – and a possible surge in promotions and the hiring of senior workers, pushing up average pay.
Reeves has already claimed there is a £22bn black hole in the public finances for this year alone, which she says was inherited from the previous government.
However, Investec’s Shaw notes there is an element of choice adding to the size of the funding shortfall.
“The Government has chosen to increase public sector pay in certain areas, which is also adding to public finance pressures,” he says.
“There was always going to be a conflict between the need to restrain public sector pay from the perspective of the public finances and of course Labour’s ties with the trade union.”
Many of the pay deals Labour has agreed since taking power are backdated to April and will add to higher borrowing figures in the months to come.
Isabel Stockton at the IFS adds that pay deals were at the “higher end of what one might have expected”.
She warns that spending so far this year looks unsustainable.
“In just one-third of this financial year the Government appears to have spent 34.1pc of what was budgeted for the whole year, whereas since comparable data began in 1997 it has never spent more than 32.9pc of the eventual total in the first third of the financial year,” adds Stockton.
“This is indicative of the scale of the pressures on departmental budgets – in some cases well and above what was budgeted for.”
Despite the financial pressures, Stockton adds that the Government has clearly signalled that it believes public sector pay deals are worth it amid recruitment struggles.
The options facing Reeves ahead of her first Budget now range from tax rises to spending cuts or higher borrowing, Stockton says.
“For any aspect of public spending that you look at, public services all across the board seem to be creaking,” she adds. “So it’s really difficult to imagine a part of spending that could be cut easily without a political cost.”
Ruth Gregory at Capital Economics says Reeves will likely seek to make tax rises to the tune of £10bn in October.
She adds: “It does feel as though the Government is slightly overplaying the gloom. There’s no doubt that the fiscal position is pretty dismal.”
But “the other recent economic developments – what’s happening on the falls in market interest rates, the falls in gilt yields, the improvement in economic growth – matter too for the Chancellor”.