Suddenly I have a 3 month notice period. What do I do?

(This is U.K. btw) So I’ve been working for the same company for the last 4 years. My contract was amended once since I passed my probation (temp to permanent), the notice period referenced in the amendment is 1 month.

I went to give my notice in, explaining that I intend on resigning in one month, to be told that I am subject to a three month notice period, as per changes to the terms and conditions which would have been published a site we access to.

No I was not informed by email/post or told to check t&c’s that I would be made to ensure 3 months, over one.

Can someone explain what to do, can I insist on one month or do I need to abide by the three?

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