British Leyland Motor Corp. v Armstrong Patents Co. is a 1986 decision of the House of Lords concerning the doctrine of non-derogation from grants. This doctrine is comparable to, but somewhat broader than, the doctrine of legal estoppel, assignor estoppel, or estoppel by deed in U.S. law. Under the doctrine of non-derogation from grants, a seller of realty or (after this decision) goods is not permitted to take any action (such as bringing an infringement action) that would lessen the value to the buyer of the thing sold.