A contract of employment is a legal agreement between the employer and the employee. It contains terms, either express or implied, which cannot lawfully be changed or varied by the employer without further agreement from the employee (either individually or through the trade union).
If the employer is thinking of changing a fundamental term of you and your colleagues’ contracts it should carry out a consultation. In other words the employer should meet with affected employees (or their representatives) and explain why it's interested in making the change. The employer has to give staff time to consider the proposal as well as to suggest alternative ways of achieving the same result (for example the employer could save money by doing x instead).
If, after receiving employees’ alternative proposals, the employer decides to go ahead with the change, it can do so. However, they must compensate you for any financial loss associated with the change, and give you sufficient notice of when the change will take effect. People affected need, after the first month of employment, to be given at least one week's notice per year of service, up to 12 weeks, or the notice due under their contracts, whichever is longer. Failing to give enough notice may make a change unlawful.
An employee can decide to accept a change, and many terms of the contract are, of course varied from time to time, by mutual consent. For example, it is quite usual for pay to be varied, usually increased, on an annual basis.
In some cases, the contract may contain an express term apparently permitting the employer to make changes from time to time. Even with such a clause, any changes would still need to be reasonable as well as introduced appropriately. Additionally, where changes are made to your contract, employers must give you written notification of the change within one month.
However, that does not change the fact that an unauthorised, one-sided variation is a breach of the contract of employment. You should consider regularly registering your opposition to the change (e.g. every month) and pursuing a claim in the employment tribunal for unlawful deduction from wages, and a declaration as to what the terms of your contract are. If the change is so fundamental that it goes to the heart of the contract, it may justify you resigning in protest. Such a resignation, if a justified response to your employer’s conduct, could be a 'constructive dismissal'.
Resigning is often, however, only advisable where you have another job to go to. An alternative would be to claim that there were two contracts (the original contract and a version amended by employer) and that the ending of the first contract amounted to an unfair dismissal. However, the compensation is small (essentially just a statutory redundancy payment - in 2011 £400 per year of service) and your previous time with the employer would no longer count towards your continuity of service.
