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In English law, the concept of legitimate expectation arises from administrative law, a branch of public law. In proceedings for judicial review, it applies the principles of fairness and reasonableness to the situation where a person has an expectation or interest in a public body retaining a long-standing practice or keeping a promise.
This concept is slowly emerging in the area of judicial review following O'Reilly v Mackman  2 AC 237. In the real world of politics, public bodies wish to be free to reform and update their practices and policies. This will often mean defeating the expectations of some affected citizens even though the new policies may actually benefit a majority of people. The problem for the courts has therefore been to decide what decision-making process public bodies should follow and how they should balance the competing interests of those likely to be affected by the proposed change.
The traditional constraint on a public body has been the test of irrationality, also know as Wednesbury unreasonableness following Associated Provincial Picture Houses Ltd v. Wednesbury Corp which stated that a decision would be unreasonable if, ". . .no reasonable authority could ever have come to it" (per Lord Greene). But if the courts are to establish a justification for a more interventionist approach, irrationality will always be defeated if the particular decision has sufficient qualities of reasonableness, i.e. it should never be irrational to prefer the good of the many to the interests of the few. Hence, when faced with claims of a legitimate expectation, the courts have begun to require public officials to adopt the same approach as in making decisions affecting fundamental human rights (now formally protected through the Human Rights Act 1998 which incorporated the European doctrine of legitimate expectation to protect the public interest in consistency and certainty through a test of proportionality).
 Emerging principles
In procedural terms, a person is entitled to a fair hearing before a decision is taken if he or she has a legitimate expectation of being heard. But the fact that a person is entitled to make representations does not, of itself, constrain public bodies which, subject to a duty not to abuse their power, are entitled to change their policies to reflect changed circumstances even though this may involve reneging on previous undertakings. If there is a substantive limitation on this right to make changes, it lies in a test of fairness where the public body's are equivalent a to a breach of contract or there have been representations that might have supported an estoppel and so caused legitimate expectations to arise. It is, of course, difficult to prove such a legitimate expectation unless fairly specific representations as to policies affecting future conduct have been made. The form of generalised understandings that ordinary citizens might have will not be sufficient for this purpose. And, even if there are legitimate expectations, there is no absolute right to have those expectations met. Fairness may require no more than a hearing or consultation before any change is finally decided and, if the citizen's expectation is real, the courts might require the public body to identifiy an overriding public interest to trump the particular expectation.
This supplements the Wednesbury approach but it may not be advancing judicial review very far since, even in cases where an estoppel might otherwise have arisen, it will be difficult to convince a court that going back on a specific representation relied on to produce detriment will be unreasonable, unfair or irrational.