In public or administrative law, a legitimate expectation is a clear, unambiguous and unqualified assurance, understood by those to whom it is given, that a particular course of action will be taken or a particular procedure will be followed.

If a failure to do so would be unfair or an abuse of power, it may be susceptible to judicial review. In determining whether a claim has been made out, it is not essential that any person for whose benefit the promise was made should have relied upon it to their detriment, but it certainly helps. Bitter disappointment is not enough, however.

Broadly, a legitimate expectation may be either

• procedural (for example, a public authority has given an unequivocal assurance that it will consult those potentially affected before changing a substantive policy, or has established a reliable practice of doing so), or
• substantive (for example, a public authority has given an unequivocal assurance that it will preserve an existing policy for a specific person or group who would be substantially affected by the change).

For a convenient summary of the relevant principles and leading cases, see United Policyholders Group v Attorney General of Trinidad and Tobago [2016] UKPC 17; [2016] 1 WLR 3383, paras 36 to 39.