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. Even if a decision will affect no existing right or legally recognized interest, the decision-maker may be bound to consult or allow a hearing to a party who has a 'legitimate expectation' that will be done.
The thrust of case law is that, whatever it is you legitimately expect (i. e.whether you expect to be consulted or whatever you expect that a certain substantive decision will not be made), the only protection that the law will give to your expectation is that the decision-makers will be required to act fairly- to observe and apply the rules of natural justice. 2 Attorney- General for Hong Kong v NG Yuen Shiu  2 AC 629: After NG Yuen Shiu entered Hong Kong illegally from Macao, the government announced a new policy of sending back illegal immigrants. Furthermore, the government announced that each illegal immigrant would be interviewed and his case treated on its merits.
Although NG Yuen was questioned and a removal order was made against him, he was not however given the opportunity to make representations. The High Court refused an order of prohibition. The Court of Appeal granted the order until NG Yuen Shiu had been given an opportunity to be heard. Judgment: Following the case of Attorney General v. Ng Yuen Shiu, the judge held that the detention was unlawful because the applicants had a legitimate expectation of being consulted before a decision was made concerning their future.
This legitimate expectation arose as a result of the clear and unequivocal promise given by the government to the applicants that their boat would be repaired and that they would be free to leave Hong Kong. The judge pointed out that there could have been other means of getting them to Japan such as enlisting the help of some voluntary or refugee organization. The judge stated that these people had legal right to expect that because of what the Government promised, they would be assisted.
* R v Secretary of State for the Home Department, ex p Khan  1 WLR 1337: The applicant, Khan, of Pakistani citizenship settled in England with his wife and requested to adopt his brother's child who was currently living in Pakistan. His brother and agreed on this and proposed adoption. A sought entry for the child was refused by the Home Office on the grounds that the Secretary of State was "not satisfied that serious and compelling family or other considerations make exclusion [of the child from the United Kingdom] undesirable.
The applicant sought judicial review of the refusal. Stephen Brown J. refused the application for judicial review. Judgment: It was held that allowing the appeal that the Secretary of State had a duty to exercise his common law discretion fairly, but in reaching his decision on a ground not included in the specified criteria the Secretary of State acted unfairly and in disregard of the applicant's legitimate expectations; and that, accordingly, the refusal of entry clearance would be quashed.
Council of Civil Service Unions v Minister for the Civil Service  AC 374: (Often referred to as the GCHQ case) In 1983 the Prime Minister (as Minister for the Civil Service) issued an instruction, with no prior consultation, peremptorily altered the conditions of civil servants employed at Government Communications Headquarters (GCHQ), a military and signals intelligence centre, should be revised so as to exclude the right of trade union membership.
The instruction was given under article 4 of the Civil Service Order in Council 1982, an Order made by virtue of what was assumed by the court to be a prerogative power, that of regulating the conduct of the civil service. What the party expected was to be consulted. This may be termed as procedural expectation: what the party legitimately expected not that a particular procedure would be followed. The applicants sought judicial review on the ground of unfairness due to failure to consult. Judgment:
The GCHQ staff had a legitimate expectation, arising from a well-established practice of consultation before their conditions of service were altered, that the minister would not make such an alteration without first consulting the staff or their trade union representatives. Glidewell J accepted these arguments and granted a declaration that the unions appealed to the House of Lords. There it was argued for the minister that the instruction was not open to review because the power to issue it had its source in the prerogative.
However, this argument was rejected by all of their Lordships. "The exercise of the prerogative power must relate to a subject matter that is 'justiciable' (Lord Scarman) and must affect the 'private rights or legitimate expectations of other persons' (Lord Diplock)"3 It was held, dismissing the appeal that (1) executive action based on common law or the use of a prerogative power was not therefore immune from review; but (2) the requirements of national security outweighed those of fairness, which was a matter for the executive to weigh and decide.
By accepting the Government's claim of national security and holding against the unions, it has been argued that "the Law Lords were too easily satisfied by some very exiguous evidence". R v Minister for Agriculture, Fisheries and Food ex p Hamble  2 All ER 714: The Ministry of Agriculture Fisheries and Food declined the applicant's request for a licence for its vessel to beam trawl for pressure stock in the North Sea because of changes in governmental policy prohibiting the aggregation of transferable licence entitlements by which method the applicant had anticipated securing the licence.
Hamble applied for judicial review of that decision, contending that by the date of the change of policy it had had a legitimate expectation that any change in licensing policy would not be such as to frustrate the completion of the process of licence aggregation for the trawler, entitling it to trawl for pressure stock in the North Sea and, further, that proper pipeline provisions should have been included in the new policy of licensing by track record in favour of fishermen who had irrevocably entered commitments to acquire beam trawl licences and who had demonstrated a real and genuine intent to do so.
Judgment: Dismissing the application, that legitimacy was a function of expectations induced by government and of policy considerations which militated against their fulfilment. Justice Sedley "In describing the two conflicting imperatives of public law "the first is that while a policy may be adopted for the exercise of a discretion it must not be applied with rigidity which excludes consideration of possible departure on individual cases…, the second is that a discretionary public law power must not be exercised arbitrarily or with partiality as between individuals or classes potentially affected by it… the line between individual consideration and inconsistency, slender enough in theory, can be imperceptible in practice".
Sedley J's approach was to encourage the adoption of a variant of the EU law model. In essence Sedley J suggested that substantive legitimate expectations can be best protected by balancing asserted individual expectations against public interest in seeking any revised policy implemented.