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A person having or claiming such an interest in unregistered land as entitles the person to object to any disposition thereof being made without the person's consent may apply to the land registrar for the registration of a caution to the effect that the cautioner is entitled to notice in the prescribed form, and to be served in the prescribed manner, of any application that may be made for the registration of the land.
Source
http://www.e-laws.gov.on.ca/html/statutes/english/elaws_statutes_90l05_ev001.htm#BK51