Rt Revd Terrence Prendergast, the Archbishop of Ottawa, was speaking after a decision from the Supreme Court of Canada to allow euthanasia in certain circumstances across the country.
The Last Rites is one of the seven Catholic sacraments, which seeks to prepare people for death and ensure they are forgiven of their sins.
The court gave the Canadian government four months to come up with legislation governing the practice nationally.
Later, a government report said some mentally ill people and children should also be allowed assisted suicide.
It also urged all taxpayer-funded healthcare to provide the option of assisted suicide.
While the Catholic Church has long opposed assisted suicide the latter recommendation would increase tensions further, as most Christian hospitals and healthcare centres are publicly funded - potentially forcing them to provide a service they fundamentally disagree with.
Bishop Prendergast told Canadian Catholic News: "Asking to be killed is gravely disordered and is a rejection of the hope that the rite calls for and tries to bring into the situation.
"Asking your priest to be present to something that is in direct contradiction to our Catholic values is not fair to the pastor.
"The rite is for people who are gravely ill or labour under the burden of years and it contains the forgiveness of sins as part of the rite.
"But we cannot be forgiven pre-emptively for something we are going to do - like ask for assisted suicide when suicide is a grave sin."
