Over the last few years the Court of Appeal have twice held that an estoppel may be used to resist a claim but, save for proprietary estoppels, cannot be used in support of a claim, and on each occasion expressed their reluctance to be bound by authority to reach this result. The purpose of this article is to show that considerations of policy, and arguably past authority, indicate that there should be no such blanket prohibition on the use of estoppels in support of a cause of action. However, the use of estoppels as an offensive weapon in litigation should be subject to two important limitations, namely (a) that, while an estoppel can support a cause of action, it cannot by itself, unless a proprietary estoppel, found a cause of action; and (b) an estoppel cannot be used to subvert statutory provisions or fundamental rules of the common law.
There can be few lawyers practising in the field of civil law who do not come across the practical implications of the law of estoppel. It sometimes seems that, when all else fails, there is the plea of last resort—can one say that there is an estoppel?
A difficulty of advising in this area is the variety of estoppels which are said to exist: common law estoppel, promissory estoppel, equitable estoppel, proprietary estoppel, estoppel by acquiescence, estoppel by encouragement, estoppel by convention, estoppel per rem judicatam
and issue estoppel are all expressions in frequent use. Sometimes a description seems to be two names for the same thing; for instance, promissory estoppel and equitable estoppel are usually taken to be the same doctrine, although proprietary estoppel also has its origins in the law as developed by the old Court of Chancery. In a recent decision of the House of Lords, Yeoman’s Row Management Ltd
Lord Scott of Foscote described proprietary estoppel as a “sub-species of promissory estoppel”. Proprietary estoppel, estoppel by acquiescence and estoppel by encouragement are usually regarded as different descriptions of the same doctrine.2
It is apparent, however, that the requirements for reliance on the various forms of estoppel are significantly different. Recent authority in the Court of Appeal, dealt with below, suggests that a common law or