BOTH Labour and the Tories are signed up to implement “serious spending cuts”, the head of a leading economic think tank has warned.

Paul Johnson, director of the Institute for Fiscal Studies (IFS), has blown apart both parties' economic claims – as he insisted austerity was guaranteed if neither party wanted to raise targets or take on more debt.

The SNP have seized on Johnson’s claim as proof both Labour and the Tories will take an axe to public budgets if either is in power after the General Election.

Responding to Tuesday’s ITV leaders debate between Rishi Sunak and Keir Starmer, Johnson said: “We don’t know what either party will do on tax over the next parliament.”

He pointed out that “both appear to be committed to one substantial tax rise” in the form of freezing personal tax allowances and thresholds for the next three years – a move worth £11 billion for the Exchequer.

And he said that Tory attacks claiming Labour’s tax policy would see significant rises were “not independently arrived at or verified”. Labour have called them “lies”.

The National: Paul Johnson – Institute For Fiscal Studies

Johnson (above) added: “Third and most importantly, it looks more likely than not that either party would need to raise taxes if they want to avoid serious spending cuts and meet the fiscal rules to which they have both signed up.”

In an article published yesterday, the IFS said it was the Government’s fiscal target – to which Labour is also signed up – that “puts a binding constraint on policy”.

READ MORE: Respected think tank says Labour and Tories 'avoiding spending cuts reality'

It requires that debt falls as a percentage of national income between four and five years out.

The think tank said while the “underlying” ambition for debt to come down was “a sensible one”, the target was “more arbitrary and gameable than most”.

And the IFS warned Labour was operating “just” within the limits of its self-imposed borrowing rules with its scaled-back green prosperity plan.

The report added that “it looks as though Labour’s new, pared-back investment plans are just about consistent with a promise to get debt falling” and warned: “It remains to be seen whether that will still be the case come the autumn.”

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Drew Hendry (above), the SNP’s economy spokesperson, said: “The IFS are clear: the economic plan proposed by both Rishi Suank and Sir Keir Starmer will lead to cuts, cuts and more cuts to core public services – underlining why it's vital to vote SNP to protect our NHS and put Scotland's interests first.

“It's high time both Westminster parties stopped being dishonest about their plans and came clean with the public on where the axe will fall when it comes to public service cuts.

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“We know that at least £18bn worth of cuts are coming down the line under the Tories and the Labour Party's spending plans, yet both Rishi Sunak and Sir Keir Starmer remain wedded to a conspiracy of silence about their joint austerity agenda.”

Scottish Tory finance spokesperson Liz Smith (below) hit back, accusing the SNP of having “some nerve given the state of our public services following their gross mismanagement of Scotland’s economy”.

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She added: “The nationalists have made Scotland the highest taxed part of the UK, and while Scots are paying more, they are undoubtedly getting less as our NHS waiting times skyrocket and our education sector remains in crisis.

“Rather than trying to pass the buck, the SNP must accept they have never protected our public services.”

Labour have been approached for comment.