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HMRC internal manual

National Insurance Manual

Class 1 NICs : Earnings of employees and office holders: Advance payments or 'subs'

Treat advance payments made regularly like any other regular payments of earnings, and include in gross pay when the payment is made.

Include occasional advance payments or ‘subs’ made out of an employee’s regular pay in the gross pay when the employer pays the balance of the normal payment and not at the time the employee receives the advance.