Your contract of employment should state details of how many hours a week and when you are required to work. An employer has every right over time to alter this for operational purposes as circumstances change. He has a business to run after all. However, this should be done in a spirit of negotiation. He is asking you to change your contract and there should be some element of quid pro quo. The best thing is for you to come to an amicable compromise whereby he gets (substantially) what he wants and you gain too. If this proves impossible and you are in a big company (as your question suggests) there may be an appeal route to branch or head office management or HR. But I'd regard this as a failure as it will do nothing for your future relationship with your boss. Do what you can to find a local solution. Good luck