This is a really interesting point and there are a number of ‘boxes’ to go down, all with different eventualities.
For example, if we take the extreme end of a limited company contractor – who pays themselves the minimum salary and dividends on top of that – a client who is keen to keep the contractor on may decide to gross-up their pay and therefore pay the NI owed. In reality this will make the cost of the contractor quite expensive to the company, if you consider the additional tax that needs to be paid.
Even in a scenario where the contractor & client may want to meet in the middle ad keep the day rate the same, the cost still increases as the client is now having to pay the contracts NI which before April they did not need too.
What we are potentially likely to see in the short-mid term is for clients to drive down day rates in order to take into account the employers NI they would now be paying.
Unfortunately this could be a real double whammy for contractors and interim professionals where your day rates have reduced, in addition to no dividends, and a higher overall income tax.