Can the employer extend the probation period?

My probation period is 90 days and my employer has told me they will extend for another 90 days due to issues noted by my line manager.

The reasons for the extension were not fair (personal rather than performance) but I was too afraid to challenge in case they terminated me on the spot.

I have also since found out they have already started looking for a replacement for me and the extension appears to have been a ploy to terminate me quickly and cheaply once this person is found.

Is any of this legal? Is there anything I can do to protect myself as the probation laws don’t seem to protect the employee?