The answer will depend on whether the bonus is treated as a gratuity, a gift which it is in the discretion of the employer to pay or not; or whether the bonus is contractual, and part of the consideration (see Chap. 2) to be given by the employer in exchange for the employee fulfilling his side of the bargain. If the former interpretation applies, then the bonus may be withdrawn by the employer at will.
The matter may be looked at in more detail. Management may claim that, in the circumstances, random or even regular payments are not to be understood as contractual, but simply as gifts, ex gratia payments or tokens of goodwill. Where extra work on the part of the employee is involved, such as increased productivity this non-contractual view of bonuses is only likely to succeed when their gratuitous nature is clear beyond any doubt, for instance if the employer inserts a note into the pay packet emphasising the discretionary nature of the bonus