Class 1 NICs: Securities: Post-acquisition benefits
Regulation 22(7) of the Social Security (Contributions) Regulations 2001
If an employee receives a benefit by virtue of the ownership of employment-related securities which is not otherwise chargeable to tax, an income tax charge arises under Chapter 4 of Part 7 of the Income Tax (Earnings and Pensions) Act 2003. The amount which counts as employment income is the actual amount or market value of the benefit.
The same amount which counts as employment income is treated as earnings liable for Class 1 NICs provided section 698 ITEPA 3003 (PAYE: special charges on employment-related securities) applies.
ERSM90030 et seq explains more about post-acquisition benefits.