Can my employer prevent an internal promotion?
Your employer is the company, either way. Your manager or supervisor may try to stand in the way of your promotion, and however wrong you feel that is, it is not illegal – there is no law requiring you get a promotion at any time, nor is there much you can do about it. The matter will be resolved by internal policies and negotiation between the two concerned parties. It will depend on who thinks you are worth more of a fight.