But it could go a step further by increasing employee NICs as well – thereby undoing some of the tax cuts made under the previous Tory government.

Increasing the main rate for those earning over £12,570 from 8pc to 9pc would generate about £5bn a year, according to the Government’s own estimates.

However Mr Stovold said the Chancellor was more likely to go after higher earners given the far-reaching consequences of her employers’ NI raid.

“The changes to the employer’s National Insurance rates and threshold that applied from this April are widely thought to result in lower earners suffering pay freezes or below inflation pay rises.

“Therefore, the Chancellor is very likely to target higher earners, as further measures directed at lower earners would be seen as a hammer blow to those who can least afford it.”

Increasing the additional rate of income tax by one percentage point would raise a mere £135m a year because of the small pool of taxpayers in this 45pc bracket.

By comparison, raising the additional rate of employee NI would raise £1.85bn. The 2pc rate currently applies to earnings over £50,270.

Change National Insurance thresholds

Alternatively the Chancellor could tweak the NI thresholds. Reducing the entry threshold by £2 a week would bring lower earners into the tax net, raising £650m over the next three years.

Shaun Moore, of wealth manager Quilter, said this could be a politically insensitive move “as it would disproportionately affect lower-income groups who are already feeling the effects of inflation”.

In addition, the Chancellor could increase the upper earnings limit for higher rate taxpayers so that more of their income attracts the 8pc NI rate as opposed to the reduced 2pc rate.

Mr Moore said: “Currently, higher earners benefit from a significant drop in their marginal NI rate once they pass the upper earnings limit. Removing or reducing this discount would raise substantial sums from high earners but could be criticised as a disincentive to high earners.”

Increasing the upper earnings limit by £10 a week could rake in £660m more over the next three years, according to HMRC’s calculations.