Answer:
This is a breach of contract. (See breach of contract)
Try to resolve the issue with your employer directly, if they are uncooperative, or do not believe that they have breached your employment contract, you technically have the legal right to take the matter to court to sue them for breaching the contract. Obviously going to court with your employer is not what you want as this will be a painful, time-consuming and costly exercise, and is only an option when you have first exhausted all manner of informal dispute resolution such as mediation and negotiation.
The first step would be to contact an employment lawyer to discuss your options, which may include sending your employer a letter of demand.
