Every employee is under an implied obligation to serve his or her employer faithfully and to be ready and willing to perform the duties set out in the express terms of his or her contract of employment. An employee who takes part in industrial action is therefore likely to be acting in serious breach of the express or implied terms of his or her contract.
The clearest example of this is where an employee withdraws his or her labour completely by going on strike, whether for a continuous period or at occasional times or on occasional days. An employee may also be acting in breach of contract if he or she refuses to carry out certain parts of his or her contractual duties by, for example, 'blacking' a particular machine or customer. An employee involved in a 'go slow' or 'work to rule' or a withdrawal of goodwill may be breaking his or her implied obligation to serve his or her employer faithfully, even if the employee is not breaching any express term of his or her contract.
There are some limited forms of industrial action that may not involve the employee breaking his or her contract. For example, an employee who refuses to work overtime may not be breaking his or her contract if the overtime is purely voluntary and the employer has no right to require the employee to work it.