It�s not illegal, but it could be against the terms and conditions of your employment and you would, therefore, be in breach of your contract of employment. Have a look at your contract of employment and see what it says. Many people have second jobs and all your employer will want to do is protect its position by making sure that any other employment you have is not detrimental to it; e.g. if your second job affects the performance of your main job (if, for instance, your second job is working nights, which then means you�re too tired to concentrate on the day job) or if the other job is working for a competitor, your employer can reasonably prevent you from taking up this other employment (or request you cease the employment if you�ve already started). Your employer shouldn�t unreasonably prevent you from taking other employment, whether during your annual leave or otherwise.