This is a budget where every letter of the Labour manifesto is being re-examined

In May, Rachel Reeves said Labour had "no plans for increased taxes".

On Wednesday, she will lay out what could be one of the biggest tax-raising budgets of modern political times.

Where those hikes fall will be dictated by one big promise sat right at the centre of the party's summer manifesto - the commitment not to put up income tax rates, national insurance or VAT for working people.

This weekend, government sources were forced to clarify that the promise was valid right up to the next election, after the education secretary appeared to fumble her lines on the matter.

Senior Whitehall figures suggested this was always obviously the case.

But this is a budget where every letter of the manifesto is now being re-examined and re-litigated to determine what tax rises do and don't breach election commitments.

As such, eagle-eyed observers may have noticed a difference in Labour's promises on corporation tax and the energy profit levy - both of which are explicitly tied to the "end of" or "entire" parliament - and the working person pledge, where there is no equivalent guarantee.

More independent figures warn the Treasury is tightening the knots on already unnecessary fiscal binds.

"Things change, unexpected events happen, we had the global pandemic... so I think to make promises which you can't realistically make doesn't make sense," said the former Bank of England governor Lord King.

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For this former monetary chief, a more honest approach would involve the Chancellor admitting the tax-lock pledge was unaffordable and putting employee national insurance back up to where it was before the last Tory government began cutting it.

While that may make more sense from an economic perspective, it would be politically unthinkable to break a manifesto pledge so brazenly and so soon after the election.

But the consequence of that is the tortuous debate currently playing out around the definition of 'working people'.

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It's also why a government whose "number one priority" is economic growth has been forced to whack businesses with a huge multi-billion pound tax rise.

"There is simply no sensible way of raising that sort of money without raising [national insurance, income tax or VAT] and the way that the Chancellor's getting round her pledges is to say it didn't apply to the employer chunk of National Insurance," said Paul Johnson of the Institute for Fiscal Studies.

The political danger is that the end result may be much the same if voters conclude that Labour has simply found loopholes and 'terms and conditions' through which to put up tax.

But the economic danger is bigger.

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Rachel Reeves is banking on growth picking up as this parliament progresses, buoyed by the huge amounts of borrowed cash she'll plough into all parts of the country in the coming months.

It's tax revenue from this hoped-for growth that will pay for the public sector in the long term.

But if that growth doesn't materialise, the government will be stuck with mounting debt repayments, cash-hungry services and a lack of money to fund any of it with.

It's then that promises made in the summer heat of the election may have to truly be bent to breaking point.