Any change like this has to be agreed by you and your employer in writing. If you can’t agree, they can make you redundant – so it really puts you in a corner!
As you’ve not been there very long, it’s hard to raise a case against them, too. I’d suggest that, if they’re forcing you to take the cut, you accept but start looking for another job. You’ll at least have some income then, until you get a new job.
Maybe there’s an employment lawyer here who can give more advice?