A contract of employment is a legal agreement between the employer and the employee. It cannot be changed without your agreement. At workplaces where a union is recognised, change should be negotiated through collective bargaining.
If you want to object to a change to your contract terms, you need to act quickly to decide what action you want to take and then make your position clear. The longer you leave it, the greater the risk that you will be taken to have accepted (that is, agreed to) the changes by putting up with them.
Speak with your rep if you have one and with co-workers if you do not, and take legal advice if necessary. A collective approach is usually the most effective way forward in this kind of scenario, for example balloting for industrial action and/or raising a collective grievance, accompanied by legal action if necessary.
If no union is recognised at your workplace, consider organising to build membership where you work, to give you and your co-workers a stronger voice when objecting to damaging cuts to contract terms. If you are not yet a union member, use our Unionfinder tool to find the most appropriate union for you.