NIM14120 - Class 1A National Insurance contributions: Exemptions from Class 1A NICs: Benefits exempt from income tax: Earnings chargeable to income tax but no income tax due

Section 10 of the Social Security Contributions and Benefits Act 1992 (SSCBA 1992)  

Class 1A NICs are due on general earnings chargeable to income tax under ITEPA 2003, even if the income tax is not payable. Chargeable does not mean the same as paid or payable. An example of this occurs where the conditions of a Double Taxation Agreement means that the UK gives credit for another country’s tax. For Class 1A NICs purposes it is the amount of the general earnings chargeable to income tax which attracts the liability, not the amount on which tax is actually paid.

For detailed information about Double Taxation Agreements you can find guidance in the Double Taxation Relief Manual.