What's Your Function?

Well, I finally feel like a lawyer.

I am writing a brief (which actually should have been written a week ago, or 5 days ago, or yesterday - when will my office learn that I need to know deadlines in advance and not a simple "well, whenever you get it done" that turns into "oh, actually, I will need that tomorrow") and I just wrote an entire paragraph arguing about the meaning of the word "may."

I know. Ridiculous.

However, I want to write another paragraph arguing about the word "unless". And I don't know if anyone will see this before I have to turn this into my boss tomorrow, but I thought I would ask. In the following sentence, is the word "unless" functioning as a conjunction or as a preposition: When a presiding officer is disqualified or it is impracticable for him to continue the hearing, another presiding officer may be assigned to continue with the case unless it is shown that substantial prejudice to the party will result therefrom. 

I feel like I should know this, but I can't really wrap my mind around the difference between the two at the moment when my mind is also trying to keep track of the other 4 arguments that I want to make in this brief. So, I'm passing it along to my grammatically gifted friends. Conjunction or Preposition? (It matters because the dictionary has one definition for "unless" as a conjunction and a different definition for "unless" as a preposition and I want to make sure that I am citing the correct one.)

Muchos Gracias!