NIM02040 | Class 1 NICs : Earnings of employees and office holders : Sums paid under a contract of employment
From HM Revenue & Customs · National Insurance Manual
Any payment which is made under the terms and conditions of a contract of employment is earnings for the purposes of NICs because this is the remuneration which an employee receives in return for providing services to their employer.
In order to decide if a payment is “contractual” (that is, made under the contract of employment) it may be necessary to consider
the terms of the written contract
the terms of any amendments to the contract contained in subsequent agreements, letters or minutes of company meetings
additional terms and conditions covered in staff handbooks, notices and circulars.
In the case of directors it may also prove necessary to look at what is contained in the company’s Memorandum and Articles of Association. This will be particularly relevant where the director has no written contract.