What to do when professor denies authoring an extremely negative letter of recommendation? I asked a professor for letters of recommendation for 7 universities. I was able to view his review for one particular university and it was extremely, unnecessarily negative. 

When I confronted him, he denied submitting this review. However, on comparing the negative letter and the actual letter that he forwarded, I found that the first paragraph of the letters are the same except that the adjectives are all negative. I have no way of knowing if it was him who submitted this. If so, there is a possibility all the universities received this negative letter. 

What do I do?
I think the question is unclear, so I'll try to cover all the scenarios.


If the professor is claiming that he submitted the positive letter and someone sabotaged it, then he should be very concerned that someone is falsifying letters. He should take the lead in helping you get to the bottom of this. If he says "oh well, someone sabotaged my letter but I don't care", then I would question his honesty.
If the professor is claiming that he did not author the letter at all, then it would seem that you've caught him in a lie. (This doesn't really make sense to me; doesn't the letter have his name on it?) Best to find another referee; if possible, you could even try to switch out the letters on your in-progress applications.
If you're not sure which of these is the case, you could express your concern that a letter was incorrectly submitted on their behalf, and ask for their help in fixing it. Or, if you have another referee, you could just try to swap them out without involving this person.