for the purposes of calcuatling NI, a director is defined as follows:-
a member of a board, or similar body where the company is managed by a board or similar body
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a single person where the company is managed by an individual, or
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if a director as defined in either of the above is accustomed to acting under the instructions of another person, that person will be a director.
As mentioned above, a Director's NI is calculated in a different manner to that of normal employees, but the result at the end of year will be the same. For directors who are paid regularly and evenly throughout the year, HMRC will allow the rules for regular employees to be adopted, thereby avoiding the jumps in levels of NI contributions that result from the strict rules, so long as a check is made at the year end.
This can produce underpayments if the director leaves part way through a tax year, and this concessional method should therefore only be used if the director and the person calculating the payroll fully understand what they are doing.