The first place to look is your employment contract. There are likely to be provisions (i) requesting you to devote the “whole [of your working] time and attention” to your employment and (ii) restricting your ability to engage in any other business activity, whether inside or outside of the contractual working hours set out in your employment contract, without obtaining prior consent from your employer. In all cases, you should try to avoid breaching the terms of your employment contract – it could result in your employer terminating your employment summarily (without notice).
There may also be restrictive covenants to consider. In the UK, these generally apply to a specific period after your employment ends but they can apply during employment too. You must also not use any of your employer’s confidential information whilst developing your startup. If your employer can demonstrate that it suffered financial loss as a result of your breach of confidentiality, it may sue you for any damages.
In addition to express contractual terms of employment, you also have implied duties to consider. Every UK employment contract contains an implied term that the employee will serve their employer with good faith and fidelity. Breaching implied terms of employment can result in termination.
It is important to note that any intellectual property created “during the course of your employment” automatically vests with your employer and this will be the case even if there is no intellectual property provision in your employment contract. As such, it is important to make sure that, whatever your startup involves, it is distinct from your employment (e.g. created outside of your contractual working hours, using your personal resources and unrelated to your employment duties) to help avoid any argument that any product created was during the course of your employment and so belongs to your employer.
Learn More: UK Founder Series: Building Your Team