Nemo iudex in causa sua

Nemo judex in causa sua (or "nemo judex in sua causa") is a Latin phrase that means, literally, "no-one is 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. In many jurisdictions 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".
This principle 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.
The phrase is credited to Sir Edward Coke in the seventeenth century, but actually attested as early as 1544.