Can an employer decrease working hours?
An employer is not allowed to suddenly decrease working hours. When your hours are clearly stated in your contract, then your employer is supposed to stick to them. Of course you and your employer are free to discuss a possible solution that you and your boss can both agree on. An employer is only allowed to solely decrease your hours when he or she takes responsibility for this reduction of hours.
NB: Are you employed on a zero hour contract basis, but have you consequently been working for the last three months on the same days and same hours? Then, your employer is not allowed to decrease your hours either.