The government introduced legislation last week to ensure that when the upper limit - above which high earners do not pay national insurance contributions - goes up, they will not get equivalent pension benefits for the extra NICs they pay. NICs have always theoretically been treated as not being a tax and as an 'insurance scheme' that qualifies payers for welfare benefits.

'Doesn't this show yet again that it's a tax?' said John Whiting of PricewaterhouseCoopers.

The upper limit is being raised to be aligned with income tax. That means taxpayers pay the 11% rate on more of their income. But they would normally qualify for a state second pension for paying more, which will now not happen as a result of the legislation.

'It points out how stupid it is that it is not a tax and that it to go through a separate bill,' added Whiting.