One aspect of the rule of natural justice is that ‘no one shall be the judge in their own case’. If a decision-maker has a financial or other interest in the outcome of the case, they cannot be, or seen to be, impartial.
The rule deals with actual bias and with the appearance of bias; hence the saying ‘Justice must not only be done, but be seen to be done’. No one should be able to allege that a decision was a fix because the decision-maker was biased, whether or not there was any truth in that allegation. The rule must be observed strictly to maintain public confidence in the decision-making process. If, for example, the applicant for a grant is known personally to the decision-maker, or the decision-maker has dealt with the applicant before and decided against the applicant or expressed a view adverse to the applicant, it may be appropriate to refer the application to a different, or more senior, official.
Article 6 of the ECHR (fair determination of civil rights) also requires that a court or tribunal must be, and have the appearance of being, impartial and independent.