Your employer can only change your contractual terms with your agreement or if it is allowed under your contract or a collective agreement you have entered into. If they try and change your terms without agreement, then that could amount to unilateral changes to your contract and, potentially, breach of contract.
If the changes are introduced forcefully then you have the following options:
1. Accept them, start working on the new terms and don't take the matter any further.
2. Start working on the new terms but making it clear to your employer (preferably in writing) that you are ‘working under protest'. This means that you do not agree to the changes but are only working them because you are forced to. In the meantime try and resolve the issue with your employer by informal discussions or through a formal grievance.
3. If Option 2 does not get you anywhere then you may wish to consider taking the final step and resigning without notice. If you do so, you would need to resign without undue delay so as not to give an impression that you had accepted the breach. Then you can claim constructive unfair dismissal in an employment tribunal. You would have to argue that your employer had committed a fundamental breach of contract and that you were left with no other option but to resign. Be aware though that you only have 3 months from the date you resign to submit your claim.
But in any case, your employer can legally introduce the changes if they give you sufficient notice to terminate your current contract and then re-employ you on a new contract, which incorporates the changes. Technically that would still be a dismissal so unless your employer can justify that there was a valid reason for the changes in the first place then you may have a case for unfair dismissal and you can make a claim in the same way in an employment tribunal.
Please press the ACCEPT button to proceed. You would then be free to ask any follow-up questions you may have. Thank you.