One of the most litigated property law issues over the past decade has been the equitable jurisdiction to set aside a mortgage transaction procured by undue influence or some other vitiating conduct.

The most common scenario is where the complainant has executed a mortgage as co-mortgagor with the principal mortgagor or has otherwise postponed his or her interest in the mortgaged property in order to secure the debts of the principal mortgagor; and, by way of a defence to an action for possession by the mortgagee, the complainant seeks to have the transaction set aside on the ground that it was procured by vitiating conduct on the part of the principal mortgagor. Barclays Bank plc v. O'Brien [1994] A.C. 180 established the structure of the defence.