What is a trustee de son tort?

In Montreal Trust Company of Canada v. Hickman et al, [2001] NFCA 42, it was noted at para. 42 that: A trustee de son tort is a person who intermeddles in the administration of a trust. As a result of the intermeddling he or she is treated, constructively, as if he or she were an express trustee and is thereby made subject to the obligations of the trustee with respect to preservation of the trust property and becomes liable to compensate, and to account to, the beneficiaries for any losses resulting from the dealing with the trust property. The trustee de son tort is also sometimes referred to as a species of constructive trustee because he or she becomes a trustee by imposition of law.

In Air Canada v. M & L Travel Limited, 1993 CanLII 33 (SCC), [1993] 3 S.C.R. 787 at p. 809-810, a case where defendants were alleged to be trustees de son tort and liable as constructive trustees for breach of trust, Iacobucci J. writing for the majority, observed that where a defendant does not assume control or possession over the property in question so that liability as a trustee de son tort could not be imposed: …strangers to the trust can also be personally liable for the breach of trust if they knowingly participate in a breach of trust… In addition to a trustee de son tort, there were traditionally therefore two ways in which a stranger to the trust could be held personally liable to the beneficiaries as a participant in the breach of trust: as one in receipt and chargeable with trust property and as one who knowingly assisted in a dishonest and fraudulent design on the part of the trustees. The former category of constructive trusteeship has been termed “knowingly receipt” or “knowingly receipt and dealing”, while the latter category has been termed “knowingly assistance.