If you have a contract with set hours can the employer change without prior notice?

The work hours set forth in your employment contract should describe exactly your hours of work (e.g. 9:00 am - 5:00 pm local time Monday through Friday). These are to be honored by both parties (Employer and Employee) as a basic tenet of the contract. It would be very unusual for such a contract not to have a clause which permits changing of any of it's terms (usually by mutual consent). If yours has a change clause review it to see whether it permits either party to change the work hours without providing advance notice. It would be equally unusual for a contract to not include such a feature. Since you question the validity of changes to your contract, it is perfectly acceptable for you to contact Human Resources (in a larger company) or your Employer (in a small business) and in a professional manner, inquire why the change is being made this way, and what terms of the contract the Employer is making them under. It's called "clarification". You may find you overlooked some fine print when signing. If not, or the answer provided by them is still not clearly supported by what you see in the contract, you should consult a lawyer or labor relations professional of your own. Before proceeding in that direction you need to weigh the consequences in terms of your relationship with the Employer, and how "winning" a point through legal action may affect your long term professional reputation.