A staff Christmas party is tax-free for your employees if it is annual, open to all employees and costs £150 or less per head. It does not have to be a Christmas party: any annual function such as a summer barbecue counts, and it also applies to online or virtual events.
Go over £150 and the whole cost becomes taxable, not just the excess. You then report it to HMRC and pay National Insurance on it.
The exemption is for staff. Entertaining clients is generally not tax deductible, so keep the two separate in your bookkeeping. If you have no employees, there is no staff party exemption to use.
Legislation and guidance: Income Tax (Earnings and Pensions) Act 2003, section 264 (annual parties and functions); HMRC Employment Income Manual EIM21690 and EIM21691 (worked examples).
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