Can you ban second jobs?

Even without a four-day week, many employees have secondary employment to supplement their income. There's no legal prohibition on employees having second jobs, but under the Working Time Regulations 1998 they must not work more than a 48-hour average working week unless they have signed an individual opt-out agreement, and this limit applies across all jobs. You are obliged to “take all reasonable steps” to ensure the weekly working time limit is complied with if there is no opt-out agreement in place.

Otherwise, whether an employee can have a second job is a contractual matter. You can insert a clause into employment contracts placing restrictions on secondary employment, but it would be usual for this to require an employee to seek your prior consent to their having a second job rather than banning them altogether, particularly for part-time and low-paid employees, as your restrictions need to be reasonable. Plus, exclusivity terms are prohibited in the contracts of both zero-hours workers and those who are entitled to net average weekly wages of no more than the lower earnings limit. If your consent is required, you again need to be reasonable in deciding whether to grant it or not. For example, it would be reasonable to refuse consent for any work which: (1) is for a competitor or is otherwise in direct or indirect competition with your business; (2) creates a conflict of interest; (3) creates a health and safety risk because it potentially impairs the employee’s ability to safely perform their duties for you due to their long working hours or insufficient breaks between jobs; or (4) means the employee would be working over 48 hours a week, unless they have signed an opt-out agreement. If you do grant consent to a second job, do also reserve the right to withdraw your consent on notice if necessary, such as where the second job is adversely affecting the employee’s attendance, timekeeping, productivity, etc.

If an employee has a second job in breach of their contract, e.g. because they did not seek your prior consent to it, that would be a potential disciplinary offence. However, if you have no objection to the second job, you could simply grant retrospective consent but do ensure you always act fairly and consistently.