Most Important X Most Importantly

[ansham]

As you can see, my doubt this time is pretty straightforward. What's the correct use and why?
"Most importantly, the candidates must apply within the next two days only."
"Most important, the candidates must apply within the next two days only."
Thank you always,
Ansham

[Tdol]

Grammatically, it should be the adverb because it is modifying the whole sentence, but you will see the second form being used.

[nyggus]

Originally Posted by ansham

"Most importantly, the candidates must apply within the next two days only."

By the way, is the only correct in this sentence? It adds nothing, I suppose; it would work in the sentence "The candidates may apply within the next two days only." Are my considerations correct?

Thanks,
Nyggus