What is the common law principle of equitable estoppel?

47. Quality Meat Packers sets out the seminal statement of the common law principle of equitable estoppel by Denning L.J. in Combe v. Combe, [1951] 1 All ER 767 (CA) at 770: The principle, as I understand it, is that where one party has, by his words or conduct, made to the other a promise or assurance which was intended to affect the legal relations between them and to be acted on accordingly, then, once the other party has taken him at his word and acted on it, the one who gave the promise or assurance cannot afterwards be allowed to revert to the previous legal relations as if no such promise or assurance had been made by him, but he must accept their legal relations subject to the qualification which he himself has so introduced, even though it is not supported in point of law by any consideration, but only by his word. [Emphasis supplied.] A representation intended by one party to affect the legal relations it has with the other party must be a representation made while the parties are in a legal relationship.2 In a labour relations context, this includes not only the period of time during which there is a contractual relationship (i.e. when a collective agreement is in effect), but the period of time when there is a legal obligation to negotiate a collective agreement. But the parties must be in some form of a legal relationship. It follows that while the long standing practice of the Employer of paying the tuition benefit may have constituted a representation to the teachers, it did not constitute a representation to the Union. Some further representation by the Employer at a time when it was within a legal relationship with the Union is necessary. In particular, in the circumstances of this case, there would need to have been a representation by the Employer within the context of collective bargaining.