When an employee terminates employment is it OK to pro rate their holiday for the months that they have completed.

Our contracts ay that in the year they terminate they will be paid for holiday for each full month they have completed. But I have recently had an employee tell me that he believes under UK law I must calculate it based on the number of weeks he worked.

I want to make sure I am paying correctly. Is there a specific rule?
Phyllis Wernikowski