Nemo judex in sua causa

Nemo judex in causa sua (or nemo judex in sua causa) is a Latin phrase that means, literally, "no-one should be a judge in his own cause." It is a principle of natural justice that no person can judge a case in which they have an interest. The rule is very strictly applied to any appearance of a possible bias, even if there is actually none: "Justice must not only be done, but must be seen to be done".

May also be called:

The legal effect of a breach of natural justice is normally to stop the proceedings and render any judgment invalid; it should be quashed or appealed, but may be remitted for a valid re-hearing.

...
Wikipedia